pack-objects: validation and documentation about unreachable options

These options are added in [1] [2] [3]. All these depend on running
rev-list internally which is normally true since they are always used
with "--all --objects" which implies --revs. But let's keep this
dependency explicit.

While at there, add documentation for them. These are mostly used
internally by git-repack. But it's still good to not chase down the
right commit message to know how they work.

[1] ca11b212eb (let pack-objects do the writing of unreachable objects
    as loose objects - 2008-05-14)
[2] 08cdfb1337 (pack-objects --keep-unreachable - 2007-09-16)
[3] e26a8c4721 (repack: extend --keep-unreachable to loose objects -
    2016-06-13)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pack-objects.txt

@@ -267,6 +267,19 @@ Unexpected missing object will raise an error.
 	locally created objects [without .promisor] and objects from the
 	promisor remote [with .promisor].)  This is used with partial clone.
 
+--keep-unreachable::
+	Objects unreachable from the refs in packs named with
+	--unpacked= option are added to the resulting pack, in
+	addition to the reachable objects that are not in packs marked
+	with *.keep files. This implies `--revs`.
+
+--pack-loose-unreachable::
+	Pack unreachable loose objects (and their loose counterparts
+	removed). This implies `--revs`.
+
+--unpack-unreachable::
+	Keep unreachable objects in loose form. This implies `--revs`.
+
 SEE ALSO
 --------
 linkgit:git-rev-list[1]

builtin/pack-objects.c

@@ -3087,6 +3087,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		fetch_if_missing = 0;
 		argv_array_push(&rp, "--exclude-promisor-objects");
 	}
+	if (unpack_unreachable || keep_unreachable || pack_loose_unreachable)
+		use_internal_rev_list = 1;
 
 	if (!reuse_object)
 		reuse_delta = 0;