Merge 'fix-slow-fetch-w-many-refs'

A regression was introduced into Git for Windows v2.13.* via a
half-finished refactoring of the refs handling. This refactoring has
been completed in the meantime, but is was only merged into upstream's
`master` branch, i.e. it was not integrated into the maintenance track.

Git for Windows is used quite a bit in enterprise settings, though,
where a large number of refs is quite common, and therefore the
regression really hurts.

So let's take this regression fix earlier than upstream Git.

This fixes #1233.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Makefile

@@ -844,6 +844,7 @@ LIB_OBJS += reflog-walk.o
 LIB_OBJS += refs.o
 LIB_OBJS += refs/files-backend.o
 LIB_OBJS += refs/iterator.o
+LIB_OBJS += refs/packed-backend.o
 LIB_OBJS += refs/ref-cache.o
 LIB_OBJS += ref-filter.o
 LIB_OBJS += remote.o

refs.c

@@ -174,6 +174,24 @@ int refname_is_safe(const char *refname)
 	return 1;
 }
 
+/*
+ * Return true if refname, which has the specified oid and flags, can
+ * be resolved to an object in the database. If the referred-to object
+ * does not exist, emit a warning and return false.
+ */
+int ref_resolves_to_object(const char *refname,
+			   const struct object_id *oid,
+			   unsigned int flags)
+{
+	if (flags & REF_ISBROKEN)
+		return 0;
+	if (!has_sha1_file(oid->hash)) {
+		error("%s does not point to a valid object!", refname);
+		return 0;
+	}
+	return 1;
+}
+
 char *refs_resolve_refdup(struct ref_store *refs,
 			  const char *refname, int resolve_flags,
 			  unsigned char *sha1, int *flags)