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.

Fixes: #183

[jerry-skydio]

Reviews in this chain: └https://github.com/Skydio/revup/pull/188 git: Add option for no_config in git shell 　└https://github.com/Skydio/revup/pull/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(-)