Move pack_refs() and friends into libgit

This moves pack_refs() and underlying functionality into the library,
to make pack-refs functionality easily available to all git programs.

Most of builtin-pack-refs.c has been moved verbatim into a new file
pack-refs.c that is compiled into libgit.a. A corresponding header
file, pack-refs.h, has also been added, declaring pack_refs() and
the #defines associated with the flags parameter to pack_refs().

This patch introduces no other changes in functionality.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-refs.h

@@ -0,0 +1,18 @@
+#ifndef PACK_REFS_H
+#define PACK_REFS_H
+
+/*
+ * Flags for controlling behaviour of pack_refs()
+ * PACK_REFS_PRUNE: Prune loose refs after packing
+ * PACK_REFS_ALL:   Pack _all_ refs, not just tags and already packed refs
+ */
+#define PACK_REFS_PRUNE 0x0001
+#define PACK_REFS_ALL   0x0002
+
+/*
+ * Write a packed-refs file for the current repository.
+ * flags: Combination of the above PACK_REFS_* flags.
+ */
+int pack_refs(unsigned int flags);
+
+#endif /* PACK_REFS_H */