Add explanatory comment for transport-helpers refs mapping.

The patch below adds a comment to fetch_with_import() explaining the
loop that saves the fetched commit names after 'git fast-import' has
done its work.  It avoids some confusion about which refs the
fast-import stream is supposed to use to write its result.

Signed-off-by: Florian Achleitner <florian.achleitner.2.6.31@gmail.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport-helper.c

@@ -444,6 +444,21 @@ static int fetch_with_import(struct transport *transport,
 	free(fastimport.argv);
 	fastimport.argv = NULL;
 
+	/*
+	 * The fast-import stream of a remote helper that advertises
+	 * the "refspec" capability writes to the refs named after the
+	 * right hand side of the first refspec matching each ref we
+	 * were fetching.
+	 *
+	 * (If no "refspec" capability was specified, for historical
+	 * reasons we default to *:*.)
+	 *
+	 * Store the result in to_fetch[i].old_sha1.  Callers such
+	 * as "git fetch" can use the value to write feedback to the
+	 * terminal, populate FETCH_HEAD, and determine what new value
+	 * should be written to peer_ref if the update is a
+	 * fast-forward or this is a forced update.
+	 */
 	for (i = 0; i < nr_heads; i++) {
 		char *private;
 		posn = to_fetch[i];