ostree/dump: Improve formatting for well-known commit metadata keys

ostree/dump: Improve formatting for well-known commit metadata keys

This follows on from commit a946c3d4, which added formatting for
well-known summary metadata keys. This commit adds it for commits.

Currently, the only well-known commit metadata key is
ostree.commit.timestamp. Formatting this correctly is especially
important, since it’s a big-endian uint64, which is completely unusable
for mere mortals when presented as a number rather than a date.

Non-formatted output can still be retrieved using the OSTREE_DUMP_RAW
flag, and the non-formatted key name is always printed for clarity.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #911
Approved by: cgwalters