git.el: Set process-environment instead of invoking env

According to the similar patch from David Kågedal [1], "this will make
it a little less posix-dependent and more efficient." However, there
are two other areas that need to replaced, namely
git-run-command-region and git-run-hooks. This patch implements the
changes of [1] onto those Emacs Lisp functions.

If unpatched, using the git port "msysgit" on Windows will require
defadvice changes as shown at [2] (also explained at 4msysgit.git
[3]).

I have tested git-run-command-region on msysgit, because this is
always called by git-commit (via git-commit-tree <- git-do-commit <-
git-commit-file). However, I could not test git-run-hooks because it
currently does not work on the Emacs Windows port. The latter reports
the hooks files as a+rw and a-x, despite msysgit and cygwin chmod
setting on the respective files.

References:
[1] f27e558643
[2] http://groups.google.com/group/msysgit/browse_thread/thread/b852fef689817707
[3] http://repo.or.cz/w/git/mingw/4msysgit.git?a=commit;h=3c30e5e87358eba7b6d7dcd6301ae8438f0c30ea

Signed-off-by: Clifford Caoile <piyo@users.sourceforge.net>
Acked-by: David Kågedal <davidk@lysator.liu.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/emacs/git.el

@@ -232,10 +232,8 @@ and returns the process output as a string, or nil if the git failed."
 
 (defun git-run-command-region (buffer start end env &rest args)
   "Run a git command with specified buffer region as input."
-  (unless (eq 0 (if env
-                    (git-run-process-region
-                     buffer start end "env"
-                     (append (git-get-env-strings env) (list "git") args))
+  (unless (eq 0 (let ((process-environment (append (git-get-env-strings env)
+                                                   process-environment)))
                   (git-run-process-region
                    buffer start end "git" args)))
     (error "Failed to run \"git %s\":\n%s" (mapconcat (lambda (x) x) args " ") (buffer-string))))
@@ -250,9 +248,8 @@ and returns the process output as a string, or nil if the git failed."
             (erase-buffer)
             (cd dir)
             (setq status
-                  (if env
-                      (apply #'call-process "env" nil (list buffer t) nil
-                             (append (git-get-env-strings env) (list hook-name) args))
+                  (let ((process-environment (append (git-get-env-strings env)
+                                                     process-environment)))
                     (apply #'call-process hook-name nil (list buffer t) nil args))))
           (display-message-or-buffer buffer)
           (eq 0 status)))))