Merge branch 'tb/reverse-midx'

An on-disk reverse-index to map the in-pack location of an object
back to its object name across multiple packfiles is introduced.

* tb/reverse-midx:
  midx.c: improve cache locality in midx_pack_order_cmp()
  pack-revindex: write multi-pack reverse indexes
  pack-write.c: extract 'write_rev_file_order'
  pack-revindex: read multi-pack reverse indexes
  Documentation/technical: describe multi-pack reverse indexes
  midx: make some functions non-static
  midx: keep track of the checksum
  midx: don't free midx_name early
  midx: allow marking a pack as preferred
  t/helper/test-read-midx.c: add '--show-objects'
  builtin/multi-pack-index.c: display usage on unrecognized command
  builtin/multi-pack-index.c: don't enter bogus cmd_mode
  builtin/multi-pack-index.c: split sub-commands
  builtin/multi-pack-index.c: define common usage with a macro
  builtin/multi-pack-index.c: don't handle 'progress' separately
  builtin/multi-pack-index.c: inline 'flags' with options

packfile.c

@@ -862,6 +862,9 @@ static void prepare_pack(const char *full_name, size_t full_name_len,
 
 	if (!strcmp(file_name, "multi-pack-index"))
 		return;
+	if (starts_with(file_name, "multi-pack-index") &&
+	    ends_with(file_name, ".rev"))
+		return;
 	if (ends_with(file_name, ".idx") ||
 	    ends_with(file_name, ".rev") ||
 	    ends_with(file_name, ".pack") ||