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>

upload-pack.c

@@ -4,6 +4,7 @@
 #include "tag.h"
 #include "object.h"
 #include "commit.h"
+#include "exec_cmd.h"
 
 static const char upload_pack_usage[] = "git-upload-pack [--strict] [--timeout=nn] <dir>";
 
@@ -60,7 +61,7 @@ static void create_pack_file(void)
 		close(0);
 		close(fd[0]);
 		close(fd[1]);
-		*p++ = "git-rev-list";
+		*p++ = "rev-list";
 		*p++ = "--objects";
 		if (create_full_pack || MAX_NEEDS <= nr_needs)
 			*p++ = "--all";
@@ -79,13 +80,13 @@ static void create_pack_file(void)
 				buf += 41;
 			}
 		*p++ = NULL;
-		execvp("git-rev-list", argv);
+		execv_git_cmd(argv);
 		die("git-upload-pack: unable to exec git-rev-list");
 	}
 	dup2(fd[0], 0);
 	close(fd[0]);
 	close(fd[1]);
-	execlp("git-pack-objects", "git-pack-objects", "--stdout", NULL);
+	execl_git_cmd("pack-objects", "--stdout", NULL);
 	die("git-upload-pack: unable to exec git-pack-objects");
 }