bug#11757: 24.1.50; vc-git calls `process-file' too many times

[Dmitry Gutov <dgutov@yandex.ru>]

When I open a file under Git version control, `vc-find-file-hook' takes
~1 second to run, even on small repositories, whereas for
Mercurial, for example, it's ~0.3 seconds.

There two two reasons for that:
1) Calling git is ~ twice as expensive as hg.
2) It's called more often.

All this under MS Windows.

Test setup:

(defadvice process-file (around time-it (program &optional infile buffer 
display &rest args) activate)
   (message "process-file %s %s" program args)
   (let ((beg (current-time)))
     (prog1 ad-do-it
       (message "%s elapsed"
                (/ (truncate (* (- (float-time (current-time))
                                   (float-time beg))
                                10000))
                   10000.0)))))

Results:

Open file from Git repo, make small modification, and save it:

   process-file git (ls-files -c -z -- INSTALL)
   0.2639 elapsed
   process-file git (symbolic-ref HEAD)
   0.1849 elapsed
   process-file git (ls-files -c -z -- INSTALL)
   0.1929 elapsed
   process-file git (diff-index -p --raw -z HEAD -- INSTALL)
   0.203 elapsed
   process-file git (rev-parse --verify HEAD)
   0.203 elapsed
   process-file git (symbolic-ref HEAD)
   0.186 elapsed
   Saving file c:/Users/gutov/vc/emacs-master/INSTALL...
   Wrote c:/Users/gutov/vc/emacs-master/INSTALL
   process-file git (ls-files -c -z -- INSTALL)
   0.217 elapsed
   process-file git (diff-index -p --raw -z HEAD -- INSTALL)
   0.233 elapsed
   process-file git (add --refresh -- INSTALL)
   0.2039 elapsed
   process-file git (symbolic-ref HEAD)
   0.21 elapsed

Note that "ls-files -c -z -- INSTALL" and "symbolic-ref HEAD" are both
called twice when the file is opened.

Do the same with Mercurial repo:

   process-file hg (--config alias.status=status --config 
defaults.status= status -A README.txt)
   0.1069 elapsed
   process-file hg (--config alias.status=status --config 
defaults.status= status -A README.txt)
   0.1099 elapsed
   process-file hg (log -l 1 --template {rev} README.txt)
   0.1019 elapsed
   Saving file c:/Users/gutov/vc/jqplot/README.txt...
   Wrote c:/Users/gutov/vc/jqplot/README.txt
   process-file hg (--config alias.status=status --config 
defaults.status= status -A README.txt)
   0.164 elapsed

Note lesser amount of calls (3 vs 6 for opening, 1 vs 4 for writing). Of
the 3, one also seems to be redundant.

In GNU Emacs 24.1.50.1 (i386-mingw-nt6.1.7601)
  of 2012-06-20 on SOL
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
  `configure --with-gcc (3.4) --cflags -IH:/Apps/System/gnuwin32/include'