Merge branch 'cc/multi-promisor'

Teach the lazy clone machinery that there can be more than one
promisor remote and consult them in order when downloading missing
objects on demand.

* cc/multi-promisor:
  Move core_partial_clone_filter_default to promisor-remote.c
  Move repository_format_partial_clone to promisor-remote.c
  Remove fetch-object.{c,h} in favor of promisor-remote.{c,h}
  remote: add promisor and partial clone config to the doc
  partial-clone: add multiple remotes in the doc
  t0410: test fetching from many promisor remotes
  builtin/fetch: remove unique promisor remote limitation
  promisor-remote: parse remote.*.partialclonefilter
  Use promisor_remote_get_direct() and has_promisor_remote()
  promisor-remote: use repository_format_partial_clone
  promisor-remote: add promisor_remote_reinit()
  promisor-remote: implement promisor_remote_get_direct()
  Add initial support for many promisor remotes
  fetch-object: make functions return an error code
  t0410: remove pipes after git commands

diff.c

@@ -25,7 +25,7 @@
 #include "packfile.h"
 #include "parse-options.h"
 #include "help.h"
-#include "fetch-object.h"
+#include "promisor-remote.h"
 
 #ifdef NO_FAST_WORKING_DIRECTORY
 #define FAST_WORKING_DIRECTORY 0
@@ -6520,8 +6520,7 @@ static void add_if_missing(struct repository *r,
 
 void diffcore_std(struct diff_options *options)
 {
-	if (options->repo == the_repository &&
-	    repository_format_partial_clone) {
+	if (options->repo == the_repository && has_promisor_remote()) {
 		/*
 		 * Prefetch the diff pairs that are about to be flushed.
 		 */
@@ -6538,8 +6537,8 @@ void diffcore_std(struct diff_options *options)
 			/*
 			 * NEEDSWORK: Consider deduplicating the OIDs sent.
 			 */
-			fetch_objects(repository_format_partial_clone,
-				      to_fetch.oid, to_fetch.nr);
+			promisor_remote_get_direct(options->repo,
+						   to_fetch.oid, to_fetch.nr);
 		oid_array_clear(&to_fetch);
 	}