bug#57807: 29.0.50; Make vc-print-branch-log able to log multiple branches

[Sean Whitton]

Hello,

vc-git-log-edit-mode is already able to properly understand git-log(1)
output which logs multiple branches.  Here's a hack to demonstrate that:

    (let ((default-directory "~/src/emacs/")
          (vc-git-log-switches
           (cons "origin/scratch/posix-spawn" vc-git-log-switches)))
      (call-process-shell-command "git branch -f tmp 4b2ca6bfc0")
      (vc-print-branch-log "tmp"))

(4b2ca6bfc0 is an old tip of master close to scratch/posix-spawn.)

An example of a case in which it is useful to produce logs like this is
when you're trying to figure out how several feature branches relate to
each other and to the trunk.

I don't think the idea of producing a log showing more than one branch
at once is fundamentally git-specific.  So I would like to extend
vc-print-branch-log to allow logging more than one branch, or
alternatively, add a vc-print-branch-logs command.  I believe that there
are two design issues, on which I'm requesting review/input:

(1) Extending the VC history functions API to permit expressing a
request for the backend to print a log of multiple branches.  We could
allow the START-REVISION argument to `print-log' to take a list of
branches, or add an additional optional argument
ADDITIONAL-START-REVISIONS.  The latter seems more robust to me.

(2) How could vc-print-branch-log prompt the user for multiple branch
names?  Magit has the user separate branch names with commas, offering
completion for each branch name.  But I suppose commas never appearing
in branch names is git-specific.  So the command could prompt the user
repeatedly for branch names, until the user hits RET to input a
zero-length branch name, indicating they've input them all.

-- 
Sean Whitton