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>
diff --git a/diff-lib.c b/diff-lib.c
index f8e936a..d85d8f3 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -231,7 +231,7 @@
 static int is_outside_repo(const char *path, int nongit, const char *prefix)
 {
 	int i;
-	if (nongit || !strcmp(path, "-") || path[0] == '/')
+	if (nongit || !strcmp(path, "-") || is_absolute_path(path))
 		return 1;
 	if (prefixcmp(path, "../"))
 		return 0;