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

[Juri Linkov]

>> Yes, please try to prepare a patch, preferably without changing the
>> established backend API, maybe by using a global variable that later
>> could be reused to confirm and edit other commands.
>
> Here's my attempt.  Let me know what you think.

Thanks, this would be one of the most useful vc features.
I tried it out, and it works nicely.  Only minor details:

Previously 'git-pull' asked for a full command including command name "git".
I know there is no need to edit the command name "git", but splitting
the command between the prompt and the minibuffer looks a little odd.

Also the previous version of 'vc-git--pushpull' sets 'compile-command'
to the edited command.  It seems this is implemented to be able to
repeat the pull command with 'g' that is bound to 'recompile'.
I don't have a good idea how to fix this.  Maybe by setting
another global variable, e.g. 'vc-last-command' after editing
the command in 'vc-do-command'.

And also 'vc-do-async-command' reports the initial unedited command
with its "Running" message string.