Exec git programs without using PATH.

The git suite may not be in PATH (and thus programs such as
git-send-pack could not exec git-rev-list).  Thus there is a need for
logic that will locate these programs.  Modifying PATH is not
desirable as it result in behavior differing from the user's
intentions, as we may end up prepending "/usr/bin" to PATH.

- git C programs will use exec*_git_cmd() APIs to exec sub-commands.
- exec*_git_cmd() will execute a git program by searching for it in
  the following directories:
	1. --exec-path (as used by "git")
	2. The GIT_EXEC_PATH environment variable.
	3. $(gitexecdir) as set in Makefile (default value $(bindir)).
- git wrapper will modify PATH as before to enable shell scripts to
  invoke "git-foo" commands.

Ideally, shell scripts should use the git wrapper to become independent
of PATH, and then modifying PATH will not be necessary.

[jc: with minor updates after a brief review.]

Signed-off-by: Michal Ostrowski <mostrows@watson.ibm.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

shell.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "quote.h"
+#include "exec_cmd.h"
 
 static int do_generic_cmd(const char *me, char *arg)
 {
@@ -7,12 +8,14 @@ static int do_generic_cmd(const char *me, char *arg)
 
 	if (!arg || !(arg = sq_dequote(arg)))
 		die("bad argument");
+	if (strncmp(me, "git-", 4))
+		die("bad command");
 
-	my_argv[0] = me;
+	my_argv[0] = me + 4;
 	my_argv[1] = arg;
 	my_argv[2] = NULL;
 
-	return execvp(me, (char**) my_argv);
+	return execv_git_cmd((char**) my_argv);
 }
 
 static struct commands {