move gbr_ctime() to a separate file and export it

The <ts> <branch> printing format is kind of useful
for default operation (not --age) as well.
use a more compact ISO-8601 presentation for timestamps
print latest commit timestamp for regular operation too
streamline timestamp printing for gbr_age()

gbr_ctime_*() has an object-taking variant as well, so we can
use that for dump_date() without duplicating the switch(type)
logic.

And apparently casting git_object *'s around is documentedly
accepted so the redundant commit lookup gets to go too.