Fix make_absolute_path() for parameters without a slash

When passing "xyz" to make_absolute_path(), make_absolute_path()
erroneously tried to chdir("xyz"), and then append "/xyz".  Instead,
skip the chdir() completely when no slash was found.

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

path.c

@@ -311,8 +311,10 @@ const char *make_absolute_path(const char *path)
 			if (last_slash) {
 				*last_slash = '\0';
 				last_elem = xstrdup(last_slash + 1);
-			} else
+			} else {
 				last_elem = xstrdup(buf);
+				*buf = '\0';
+			}
 		}
 
 		if (*buf) {