exec_cmd.c: use find_last_dir_sep() for code simplification

We are trying to extract dirname from argv0 in the git_extract_argv0_path().
But in the same time, the <git-compat-util.h> provides find_last_dir_sep()
to get dirname from a given path.  Let's use it instead of loop for the code
simplification.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

exec_cmd.c

@@ -43,12 +43,10 @@ const char *git_extract_argv0_path(const char *argv0)
 
 	if (!argv0 || !*argv0)
 		return NULL;
-	slash = argv0 + strlen(argv0);
 
-	while (argv0 <= slash && !is_dir_sep(*slash))
-		slash--;
+	slash = find_last_dir_sep(argv0);
 
-	if (slash >= argv0) {
+	if (slash) {
 		argv0_path = xstrndup(argv0, slash - argv0);
 		return slash + 1;
 	}