Documentation: distinguish between ref and offset deltas in pack-format

eb32d236 introduced the OBJ_OFS_DELTA object that uses a relative offset to
identify the base object instead of the 20-byte SHA1 reference. The pack file
documentation only mentions the SHA1 based reference in its description of the
deltified object entry.

Update the pack format documentation to clarify that the deltified object
representation refers to its base using either a relative negative offset or
the absolute SHA1 identifier.

Signed-off-by: Stefan Saasen <ssaasen@atlassian.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
fast-export: fix argument name in error messages

The --signed-tags argument is plural, while error messages referred
to --signed-tag (singular).  Tweak error messages to correspond to the
argument.

Signed-off-by: Paul Price <price@astro.princeton.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>