Add a simple LRU cache to common git cmds

[jerry-skydio]

Simple commands such as ref existence, id lookup,
and distance lookup are pretty common and could be
called with the same args at a higher level such that
its difficult to dedupe between calls. To speed things
up, add a lru_cache decorator so that later calls are
looked up. We don't add this to everything since
more complex functions are unlikely to be called with
the same args multiple times.

These calls are mostly pure functions since revup
generally doesn't change the git database. There are
a few exceptions to that, when it runs "reset" for certain
commands. In these cases, we empty all caches just to be safe.

Topic: cache
Reviewers: brian-k

[jerry-skydio]

Reviews in this chain:
└#108 Add a simple LRU cache to common git cmds
　└#109 git: Switch cherry-pick backend to git merge-tree

[jerry-skydio]

#	head	base	diff	date	summary
0	2a31f7b1	9a01d118	diff	Apr 4 22:23 PM	3 files changed, 10 insertions(+), 8 deletions(-)
1	3905dadc	9a01d118	diff	Apr 5 0:19 AM	1 file changed, 7 insertions(+)
2	cb9c8684	9a01d118	diff	Apr 5 0:40 AM	3 files changed, 13 insertions(+), 2 deletions(-)
3	5dfb7574	9a01d118	diff	Apr 5 0:41 AM	0 files changed
4	02ea972a	9a01d118	diff	Apr 5 1:10 AM	1 file changed, 7 insertions(+), 7 deletions(-)
5	ee23a8f5	9a01d118	diff	Apr 5 1:11 AM	0 files changed
6	5999441f	9a01d118	diff	Apr 5 4:09 AM	0 files changed
7	ee23a8f5	9a01d118	diff	Apr 5 22:49 PM	0 files changed