amend: Fix last-touched functionality

[jerry-skydio]

last-touched will amend the file into the correct
commit, but incorrectly then removes the changes immediately
when replaying the rest of the stack on top. This was not
caught by tests because those only checked the exact commit
and file, without checking that the rest of the stack is
correct.

Fix this by reimplementing with the merge-tree engine, which
is more modern and faster anyway. Update tests to catch this case.

[jerry-skydio]

Reviews in this chain:
└#267 amend: Fix last-touched functionality
　└#276 amend: Fix last-touched when deleting files
　　└#284 git: Use -z for null separated parsing

[jerry-skydio]

#	head	base	diff	date	summary
0	8c729e81	189dacb5	diff	Jun 25 13:06 PM	3 files changed, 75 insertions(+), 24 deletions(-)
1	9279b2fe	29c140d2	diff	Jun 25 13:35 PM	0 files changed
2	6f491aac	d8008423	rebase	Jun 26 10:38 AM	0 files changed
3	72e5eae6	d8008423	diff	Jul 23 14:41 PM	1 file changed, 29 insertions(+), 12 deletions(-)
4	0f68ad0f	d8008423	diff	Jul 23 14:59 PM	2 files changed, 29 insertions(+), 30 deletions(-)
5	d9e4421c	d8008423	diff	Jul 23 16:01 PM	1 file changed, 7 insertions(+), 1 deletion(-)
6	f3120035	bb92346f	rebase	Jul 27 13:47 PM	0 files changed
7	d6fce997	6b948064	rebase	Jul 27 13:52 PM	0 files changed