pack-write.c: prepare to write 'pack-*.rev' files

This patch prepares for callers to be able to write reverse index files
to disk.

It adds the necessary machinery to write a format-compliant .rev file
from within 'write_rev_file()', which is called from
'finish_tmp_packfile()'.

Similar to the process by which the reverse index is computed in memory,
these new paths also have to sort a list of objects by their offsets
within a packfile. These new paths use a qsort() (as opposed to a radix
sort), since our specialized radix sort requires a full revindex_entry
struct per object, which is more memory than we need to allocate.

The qsort is obviously slower, but the theoretical slowdown would
require a repository with a large amount of objects, likely implying
that the time spent in, say, pack-objects during a repack would dominate
the overall runtime.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack.h

@@ -42,6 +42,8 @@ struct pack_idx_option {
 	/* flag bits */
 #define WRITE_IDX_VERIFY 01 /* verify only, do not write the idx file */
 #define WRITE_IDX_STRICT 02
+#define WRITE_REV 04
+#define WRITE_REV_VERIFY 010
 
 	uint32_t version;
 	uint32_t off32_limit;
@@ -91,6 +93,8 @@ struct ref;
 
 void write_promisor_file(const char *promisor_name, struct ref **sought, int nr_sought);
 
+const char *write_rev_file(const char *rev_name, struct pack_idx_entry **objects, uint32_t nr_objects, const unsigned char *hash, unsigned flags);
+
 /*
  * The "hdr" output buffer should be at least this big, which will handle sizes
  * up to 2^67.