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>

receive-pack.c

@@ -6,7 +6,7 @@
 
 static const char receive_pack_usage[] = "git-receive-pack <git-dir>";
 
-static const char unpacker[] = "git-unpack-objects";
+static char *unpacker[] = { "unpack-objects", NULL };
 
 static int report_status = 0;
 
@@ -257,7 +257,7 @@ static void read_head_info(void)
 
 static const char *unpack(int *error_code)
 {
-	int code = run_command(unpacker, NULL);
+	int code = run_command_v_opt(1, unpacker, RUN_GIT_CMD);
 
 	*error_code = 0;
 	switch (code) {