git: Use merge-tree in patchsets

[jerry-skydio]

Previously patchsets used a hacky invocation of update-index
to overlay changed files from old tree to new tree. In addition
to being slower than necessary, this has the downside of showing
upstream changes in the diff, even those that were not changed
in the given review upload.

Using git merge-tree for this instead is cleaner, faster, and better.
We do this by "cherry-picking" the old branch with git merge-tree, but providing -X ours as
the strategy. This provides a tree that looks like the new base with the old changes applied.
For cases where the old changes don't apply cleanly, the ours strategy prevents conflicts by
taking the version in the old tree. Although this can be a bit confusing when looking at the
file as a whole, it provides a succinct way to view a difference between the old version and
new version.

Topic: mergetreepatch
Fixes: #183

[jerry-skydio]

Reviews in this chain:
└#187 git: Use merge-tree in patchsets

[jerry-skydio]

#	head	base	diff	date	summary
0	b427e643	740c5cdb	diff	Jul 3 12:15 PM	1 file changed, 21 insertions(+), 36 deletions(-)
1	672d68bd	740c5cdb	diff	Jul 3 12:53 PM	1 file changed, 1 insertion(+), 2 deletions(-)
2	6f599e77	740c5cdb	diff	Jul 3 13:03 PM	1 file changed, 7 insertions(+), 17 deletions(-)
3	61afb41d	740c5cdb	diff	Jul 3 13:04 PM	0 files changed
4	b4fe6239	740c5cdb	diff	Jul 3 13:25 PM	1 file changed, 1 insertion(+), 1 deletion(-)
5	775056df	740c5cdb	diff	Jul 3 13:58 PM	1 file changed, 14 deletions(-)
6	21f00572	1d315c52	diff	Jul 3 14:06 PM	0 files changed
7	cbf755de	cb8fa697	diff	Jul 3 14:08 PM	1 file changed, 3 insertions(+), 4 deletions(-)
8	62ebf0b9	64fe866f	diff	Jul 8 16:07 PM	0 files changed
9	6729d4f5	4e463646	diff	Sep 4 15:59 PM	1 file changed, 11 insertions(+), 2 deletions(-)
10	e65453bd	0ffe5352	rebase	Nov 5 13:37 PM	0 files changed
11	37894add	920df710	diff	Dec 6 17:15 PM	1 file changed, 3 insertions(+), 3 deletions(-)
12	d7e65a1c	c7b92e83	diff	Dec 28 17:20 PM	0 files changed
13	9baa70ad	bdffc6d6	diff	Dec 28 17:25 PM	0 files changed
14	7a0a131f	5d4048f9	diff	Dec 28 17:27 PM	1 file changed, 1 insertion(+)
15	ee7227da	bdffc6d6	diff	Dec 28 17:31 PM	1 file changed, 2 deletions(-)
16	204fb646	c7b92e83	diff	Dec 28 17:32 PM	0 files changed

[aaron-skydio]

Cool