negotiator/noop: add noop fetch negotiator

Add a noop fetch negotiator. This is introduced to allow partial clones
to skip the unneeded negotiation step when fetching missing objects
using a "git fetch" subprocess. (The implementation of spawning a "git
fetch" subprocess will be done in a subsequent patch.) But this can also
be useful for end users, e.g. as a blunt fix for object corruption.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-negotiator.c

@@ -2,6 +2,7 @@
 #include "fetch-negotiator.h"
 #include "negotiator/default.h"
 #include "negotiator/skipping.h"
+#include "negotiator/noop.h"
 #include "repository.h"
 
 void fetch_negotiator_init(struct repository *r,
@@ -13,6 +14,10 @@ void fetch_negotiator_init(struct repository *r,
 		skipping_negotiator_init(negotiator);
 		return;
 
+	case FETCH_NEGOTIATION_NOOP:
+		noop_negotiator_init(negotiator);
+		return;
+
 	case FETCH_NEGOTIATION_DEFAULT:
 	default:
 		default_negotiator_init(negotiator);