bug#8756: 23.3; vc-git.el doesn't use --follow argument in vc-git-print-log

[Steinar Bang <sb@dod.no>]

>>>>> Dan Nicolaescu <dann@gnu.org>:

> Is it possible to compute the file name just the sha1?  That would allow
> us to avoid using --name-only.

Maybe.

From what I learnt when I asked a question on the git list[1], here is
an example on the rename commit:

 1. first do a diff-tree on the commit where the rename takes place
    #+begin_example
      sb@somehost:~$ git diff-tree -M a50a042e5af770928eaa8b636517d66bc2a13ee7
      a50a042e5af770928eaa8b636517d66bc2a13ee7
      :100644 100644 f6a9be87a391c29bf58467e6cda8ffc2375a1471 e1dadc8553ec388b790a4c4dd3f3b6e498f9eedc M      .emacs
      :040000 040000 beff95d4c2f3d62e7d41f061738b86e3187ec887 155295fd0ad500419a0b5e2d4c571334675fbae0 M      apps
    #+end_example
    As you can see, nothing obvious here, so you have to try "diff -M"
    on the different sha1 pairs.  I, as a human, can see that the
    first one isn't an obvious candidate.  But a program would have to
    try all of them
 2. do "diff -M" on the sha1 pairs of diff-tree results to find the
    renames
    #+begin_example
      sb@somehost:~$ git diff -M f6a9be87a391c29bf58467e6cda8ffc2375a1471 e1dadc8553ec388b790a4c4dd3f3b6e498f9eedc
      diff --git a/f6a9be87a391c29bf58467e6cda8ffc2375a1471 b/e1dadc8553ec388b790a4c4dd3f3b6e498f9eedc
      index f6a9be8..e1dadc8 100644
      --- a/f6a9be87a391c29bf58467e6cda8ffc2375a1471
      +++ b/e1dadc8553ec388b790a4c4dd3f3b6e498f9eedc
      @@ -63,6 +63,8 @@ to be something different.")
             (push (concat apps-dir "/share/emacs21/site-lisp") load-path))
         (if running-emacs22
             (push (concat apps-dir "/share/emacs22/site-lisp") load-path))
      +  (if running-emacs23
      +      (push (concat apps-dir "/share/emacs23/site-lisp") load-path))
         (setq apps-bin (concat apps-dir "/bin"))
         (when (file-directory-p apps-elisp)
           (add-to-list 'load-path apps-elisp))
      sb@somehost:~$ git diff -M beff95d4c2f3d62e7d41f061738b86e3187ec887 155295fd0ad500419a0b5e2d4c571334675fbae0
      diff --git a/share/emacs/site-lisp/vc-git.el b/share/emacs23/site-lisp/vc-git.el
      similarity index 100%
      rename from share/emacs/site-lisp/vc-git.el
      rename to share/emacs23/site-lisp/vc-git.el
    #+end_example

So... I guess is possible.  But you have to work your way back through
each commit in the history, and look for "rename to" and "rename from",
and keep track of all renames as you work your way backwards.

Wnen I think of it, it would be simpler to do a "git log --follow
--name-only" and just regexp out the paths and commits and get an alist
from commit to path, or something like that...?  Actually, you probably
don't have to create that alist, just doing "git log --follow
--name-only" just to get the path when you need it, maybe...?

[1] Question asked on the git list
    http://thread.gmane.org/gmane.comp.version-control.git/186009/focus=186032