Replace patch-id rebase detection with merge-tree comparison

[jerry-skydio]

Patch-id can produce false matches on large blocks of repeated text.
Instead, apply each local commit onto the remote commit's parent via
merge-tree and compare the resulting tree to the remote tree.

This also fixes a long standing corner case where a change could be
marked non-rebase even though applying that commit would result in the
same thing. This was because diffs that are already applied are accepted
as no-ops, but still affect the patch id.

Fixes: #182
Fixes: #174

[jerry-skydio]

Reviews in this chain:
└#246 Replace patch-id rebase detection with merge-tree comparison

[jerry-skydio]

#	head	base	diff	date	summary
0	123edbe6	0ef7ecb0	diff	May 7 19:17 PM	2 files changed, 53 insertions(+), 68 deletions(-)
1	0253b888	2f428c79	diff	May 11 14:21 PM	4 files changed, 107 insertions(+), 58 deletions(-)
2	6dc613e5	b1104d77	rebase	May 18 18:44 PM	0 files changed
3	162c8ba7	8686667c	rebase	May 18 18:51 PM	0 files changed
4	cfc0a80e	f29bfafb	diff	May 18 18:53 PM	0 files changed