pack-objects: create new name-hash algorithm (#5157)

This is an updated version of gitgitgadget#1785, intended for early
consumption into Git for Windows.

The idea here is to add a new `--full-name-hash` option to `git
pack-objects` and `git repack`. This adjusts the name-hash value used
for finding delta bases in such a way that uses the full path name with
a lower likelihood of collisions than the default name-hash algorithm.
In many repositories with name-hash collisions and many versions of
those paths, this can significantly reduce the size of a full repack. It
can also help in certain cases of `git push`, but only if the pack is
already artificially inflated by name-hash collisions; cases that find
"sibling" deltas as better choices become worse with `--full-name-hash`.

Thus, this option is currently recommended for full repacks of large
repos, and on client machines without reachability bitmaps.

Some care is taken to ignore this option when using bitmaps, either
writing bitmaps or using a bitmap walk during reads. The bitmap file
format contains name-hash values, but no way to indicate which function
is used, so compatibility is a concern for bitmaps. Future work could
explore this idea.

After this PR is merged, then the more-involved `--path-walk` option may
be considered.

Author: dscho

builtin/pack-objects.c

@@ -4439,7 +4439,7 @@ int cmd_pack_objects(int argc,
 				N_("protocol"),
 				N_("exclude any configured uploadpack.blobpackfileuri with this protocol")),
 		OPT_BOOL(0, "full-name-hash", &use_full_name_hash,
-			 N_("optimize delta compression across identical path names over time")),
+			 N_("(EXPERIMENTAL!) optimize delta compression across identical path names over time")),
 		OPT_END(),
 	};
 

builtin/repack.c

@@ -1209,7 +1209,7 @@ int cmd_repack(int argc,
 		OPT_BOOL('F', NULL, &po_args.no_reuse_object,
 				N_("pass --no-reuse-object to git-pack-objects")),
 		OPT_BOOL(0, "full-name-hash", &po_args.full_name_hash,
-				N_("pass --full-name-hash to git-pack-objects")),
+				N_("(EXPERIMENTAL!) pass --full-name-hash to git-pack-objects")),
 		OPT_NEGBIT('n', NULL, &run_update_server_info,
 				N_("do not run git-update-server-info"), 1),
 		OPT__QUIET(&po_args.quiet, N_("be quiet")),