amend: Refresh committer date

[jerry-skydio]

Previously amend would keep all timestamps the same. However
we want to refresh just the commit timestamp on any commits
being modified. Commits that are simply recreated on top do
not have their timestamps modified.

[jerry-skydio]

Reviews in this chain:
└#262 amend: Refresh committer date
　└#267 amend: Fix last-touched functionality

[jerry-skydio]

#	head	base	diff	date	summary
0	bd1009e1	e03937b2	diff	Jun 19 13:41 PM	3 files changed, 65 insertions(+), 3 deletions(-)
1	189dacb5	e03937b2	diff	Jun 25 13:06 PM	1 file changed, 5 insertions(+), 2 deletions(-)