fetch: fetch objects by their exact SHA-1 object names

Teach "git fetch" to accept an exact SHA-1 object name the user may
obtain out of band on the LHS of a pathspec, and send it on a "want"
message when the server side advertises the allow-tip-sha1-in-want
capability.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/remote.h b/remote.h
index 251d8fd..f7b08f1 100644
--- a/remote.h
+++ b/remote.h
@@ -62,6 +62,7 @@
 	unsigned force : 1;
 	unsigned pattern : 1;
 	unsigned matching : 1;
+	unsigned exact_sha1 : 1;
 
 	char *src;
 	char *dst;