cat-file: print tags raw for "cat-file -p"

cat-file: print tags raw for "cat-file -p"

When "cat-file -p" prints commits, it shows them in their
raw format, since git's format is already human-readable.
For tags, however, we print the whole thing raw except for
one thing: we convert the timestamp on the tagger line into a
human-readable date.

This dates all the way back to a0f15fa (Pretty-print tagger
dates, 2006-03-01). At that time there was no other way to
pretty-print a tag.  These days, however, neither of those
matters much. The normal way to pretty-print a tag is with
"git show", which is much more flexible than "cat-file -p".

Commit a0f15fa also built "verify-tag --verbose" (and
subsequently "tag -v") around the "cat-file -p" output.
However, that behavior was lost in commit 62e09ce (Make git
tag a builtin, 2007-07-20), and we went back to printing
the raw tag contents. Nobody seems to have noticed the bug
since then (and it is arguably a saner behavior anyway, as
it shows the actual bytes for which we verified the
signature).

Let's drop the tagger-date formatting for "cat-file -p". It
makes us more consistent with cat-file's commit
pretty-printer, and as a bonus, we can drop the hand-rolled
tag parsing code in cat-file (which happened to behave
inconsistently with the tag pretty-printing code elsewhere).

This is a change of output format, so it's possible that
some callers could considered this a regression. However,
the original behavior was arguably a bug (due to the
inconsistency with commits), likely nobody was relying on it
(even we do not use it ourselves these days), and anyone
relying on the "-p" pretty-printer should be able to expect
a change in the output format (i.e., while "cat-file" is
plumbing, the output format of "-p" was never guaranteed to
be stable).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>