Replace most uses of strncmp() with prefixcmp()

This is a preparation for replacing all prefix checks with either
strip_prefix() or starts_with() when Git 1.8.6 is released.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

ui-summary.c

@@ -116,7 +116,7 @@ static char* append_readme_path(const char *filename, const char *ref, const cha
 	if (!ref) {
 		resolved_base = realpath(base_dir, NULL);
 		resolved_full = realpath(full_path, NULL);
-		if (!resolved_base || !resolved_full || strncmp(resolved_base, resolved_full, strlen(resolved_base))) {
+		if (!resolved_base || !resolved_full || prefixcmp(resolved_full, resolved_base)) {
 			free(full_path);
 			full_path = NULL;
 		}