bug#38044: 27.0.50; There should be an easier way to look at a specific vc commit

[Juri Linkov]

>> @@ -2416,7 +2416,10 @@ vc-log-internal-common
>>        (let ((inhibit-read-only t))
>>      ;; log-view-mode used to be called with inhibit-read-only bound
>>      ;; to t, so let's keep doing it, just in case.
>> -    (vc-call-backend backend 'log-view-mode)
>> +    (vc-call-backend backend
>> +                         (if (eq type 'with-diff)
>> +                             'region-history-mode
>> +                           'log-view-mode))
>
> Can we avoid calling region-history-mode if the backend doesn't
> implement it?  Or do you plan on adding that to all backends?

A new patch avoids calling region-history-mode
if the backend doesn't implement it:

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index ca4c66a06d..71307cdffd 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1161,6 +1160,8 @@ vc-git-print-log
                                                             "HEAD"
                                                           limit)))
                     (list start-revision)))
+                (when (eq vc-log-view-type 'with-diff)
+                  (list "-p"))
                '("--")))))))
 
 (defun vc-git-log-outgoing (buffer remote-location)
@@ -1226,7 +1227,7 @@ vc-git-log-view-mode
   (set (make-local-variable 'log-view-file-re) regexp-unmatchable)
   (set (make-local-variable 'log-view-per-file-logs) nil)
   (set (make-local-variable 'log-view-message-re)
-       (if (not (memq vc-log-view-type '(long log-search)))
+       (if (not (memq vc-log-view-type '(long log-search with-diff)))
           (cadr vc-git-root-log-format)
         "^commit *\\([0-9a-z]+\\)"))
   ;; Allow expanding short log entries.
@@ -1235,7 +1236,7 @@ vc-git-log-view-mode
     (set (make-local-variable 'log-view-expanded-log-entry-function)
         'vc-git-expanded-log-entry))
   (set (make-local-variable 'log-view-font-lock-keywords)
-       (if (not (memq vc-log-view-type '(long log-search)))
+       (if (not (memq vc-log-view-type '(long log-search with-diff)))
           (list (cons (nth 1 vc-git-root-log-format)
                       (nth 2 vc-git-root-log-format)))
         (append
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 0d29c80d02..d0d2c39ac3 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -2361,7 +2361,7 @@ vc-print-log-setup-buttons
                         'help-echo "Show the log again, including all 
entries")))
 
 (defun vc-print-log-internal (backend files working-revision
-                                      &optional is-start-revision limit)
+                                      &optional is-start-revision limit type)
   "For specified BACKEND and FILES, show the VC log.
 Leave point at WORKING-REVISION, if it is non-nil.
 If IS-START-REVISION is non-nil, start the log from WORKING-REVISION
@@ -2377,7 +2377,7 @@ vc-print-log-internal
          (shortlog (not (null (memq (if dir-present 'directory 'file)
                                     vc-log-short-style))))
        (buffer-name "*vc-change-log*")
-         (type (if shortlog 'short 'long)))
+         (type (or type (if shortlog 'short 'long))))
       (vc-log-internal-common
        backend buffer-name files type
        (lambda (bk buf _type-arg files-arg)
@@ -2393,7 +2393,7 @@ vc-print-log-internal
            (vc-call-backend bk 'show-log-entry working-revision)))
        (lambda (_ignore-auto _noconfirm)
         (vc-print-log-internal backend files working-revision
-                              is-start-revision limit)))))
+                              is-start-revision limit type)))))
 
 (defvar vc-log-view-type nil
   "Set this to differentiate the different types of logs.")
@@ -2416,7 +2416,12 @@ vc-log-internal-common
       (let ((inhibit-read-only t))
        ;; log-view-mode used to be called with inhibit-read-only bound
        ;; to t, so let's keep doing it, just in case.
-       (vc-call-backend backend 'log-view-mode)
+       (vc-call-backend backend
+                         (if (and (eq type 'with-diff)
+                                  (vc-find-backend-function
+                                   backend 'region-history-mode))
+                             'region-history-mode
+                           'log-view-mode))
        (set (make-local-variable 'log-view-vc-backend) backend)
        (set (make-local-variable 'log-view-vc-fileset) files)
        (set (make-local-variable 'revert-buffer-function)
@@ -2475,13 +2480,26 @@ vc-print-log
     (vc-print-log-internal backend files working-revision nil limit)))
 
 ;;;###autoload
-(defun vc-print-root-log (&optional limit)
+(defun vc-print-root-log (&optional limit revision)
   "List the change log for the current VC controlled tree in a window.
 If LIMIT is non-nil, it should be a number specifying the maximum
 number of revisions to show; the default is `vc-log-show-limit'.
-When called interactively with a prefix argument, prompt for LIMIT."
+When called interactively with a prefix argument, prompt for LIMIT.
+When the prefix argument is a number, use it as LIMIT.
+A special case is when the prefix argument is 1, in this case
+it asks for the revision and shows it with its diff."
   (interactive
    (cond
+    ((eq current-prefix-arg 1)
+     (let* ((default (thing-at-point 'word))
+           (revision (read-string
+                      (if default
+                          (format "Revision to show (default %s): " default)
+                        "Revision to show: ")
+                      nil nil default)))
+       (list 1 revision)))
+    ((numberp current-prefix-arg)
+     (list current-prefix-arg))
     (current-prefix-arg
      (let ((lim (string-to-number
                 (read-from-minibuffer
@@ -2492,9 +2510,11 @@ vc-print-root-log
        (list lim)))
     (t
      (list (when (> vc-log-show-limit 0) vc-log-show-limit)))))
-  (let ((backend (vc-deduce-backend))
-       (default-directory default-directory)
-       rootdir)
+  (let* ((backend (vc-deduce-backend))
+        (default-directory default-directory)
+        (with-diff (and (eq limit 1) revision))
+        (vc-log-short-style (unless with-diff vc-log-short-style))
+        rootdir)
     (if backend
        (setq rootdir (vc-call-backend backend 'root default-directory))
       (setq rootdir (read-directory-name "Directory for VC root-log: "))
@@ -2502,7 +2522,8 @@ vc-print-root-log
       (unless backend
         (error "Directory is not version controlled")))
     (setq default-directory rootdir)
-    (vc-print-log-internal backend (list rootdir) nil nil limit)))
+    (vc-print-log-internal backend (list rootdir) revision revision limit
+                           (when with-diff 'with-diff))))
 
 ;;;###autoload
 (defun vc-print-branch-log (branch)