amend: Add more detail to the reflog action

[jerry-skydio]

Previously, amend had a customized reflog message, but
it was static and did not have detail on which change was
amended. This made looking through the reflog pretty confusing
if amend was used many times.

Add additional detail to show whether drop/insert was used and
which commit was targeted with the amend. We don't need to
show the new HEAD because this is already in the reflog.

[jerry-skydio]

Reviews in this chain:
└#150 amend: Add more detail to the reflog action

[jerry-skydio]

#	head	base	diff	date	summary
0	e8d675c9	d7f409ca	diff	Mar 11 12:49 PM	1 file changed, 7 insertions(+), 1 deletion(-)
1	c9a179eb	d7f409ca	diff	Mar 11 12:51 PM	1 file changed, 4 insertions(+), 5 deletions(-)