Add for_each_remote() function, and extend remote_find_tracking()

The function for_each_remote() does exactly what the name
suggests.

The function remote_find_tracking() was extended to be able to
search remote refs for a given local ref.  The caller sets
either src or dst (but not both) in the refspec parameter, and
remote_find_tracking() will fill in the other and return 0.

Both changes are required for the next step: simplification of
git-branch's --track functionality.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/remote.h b/remote.h
index 080b7da..17b8b5b 100644
--- a/remote.h
+++ b/remote.h
@@ -20,13 +20,16 @@
 
 struct remote *remote_get(const char *name);
 
+typedef int each_remote_fn(struct remote *remote, void *priv);
+int for_each_remote(each_remote_fn fn, void *priv);
+
 int remote_has_uri(struct remote *remote, const char *uri);
 
 struct refspec {
 	unsigned force : 1;
 	unsigned pattern : 1;
 
-	const char *src;
+	char *src;
 	char *dst;
 };