Use is_absolute_path() in diff-lib.c, lockfile.c, setup.c, trace.c

Using the helper function to test for absolute paths makes porting easier.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lockfile.c

@@ -94,7 +94,7 @@ static char *resolve_symlink(char *p, size_t s)
 			return p;
 		}
 
-		if (link[0] == '/') {
+		if (is_absolute_path(link)) {
 			/* absolute path simply replaces p */
 			if (link_len < s)
 				strcpy(p, link);