git wrapper: Make while loop more reader-friendly

It is not a good practice to prefer performance over readability in
something as performance uncritical as finding the trailing slash
of argv[0].

So avoid head-scratching by making the loop user-readable, and not
hyper-performance-optimized.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -428,9 +428,8 @@ int main(int argc, const char **argv)
 	 * name, and the dirname as the default exec_path
 	 * if we don't have anything better.
 	 */
-	do
-		--slash;
-	while (cmd <= slash && !is_dir_sep(*slash));
+	while (cmd <= slash && !is_dir_sep(*slash))
+		slash--;
 	if (cmd <= slash) {
 		*slash++ = 0;
 		git_set_argv0_path(cmd);