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>
diff --git a/Makefile b/Makefile
index b003e3e..adbe442 100644
--- a/Makefile
+++ b/Makefile
@@ -354,6 +354,7 @@
 LIB_H += mailmap.h
 LIB_H += object.h
 LIB_H += pack.h
+LIB_H += pack-refs.h
 LIB_H += pack-revindex.h
 LIB_H += parse-options.h
 LIB_H += patch-ids.h
@@ -429,6 +430,7 @@
 LIB_OBJS += name-hash.o
 LIB_OBJS += object.o
 LIB_OBJS += pack-check.o
+LIB_OBJS += pack-refs.o
 LIB_OBJS += pack-revindex.o
 LIB_OBJS += pack-write.o
 LIB_OBJS += pager.o