Cherry-pick can fetch base-branch

[aaron-skydio]

Useful if cherry-picking from a relative topic chain.

It would be nice if we could automatically cherry-pick just the commits
from a particular topic without needing to specify the previous topic as
the base-branch, but that would require either fetching "more stuff"
from the remote and doing some guessing, or querying the github API,
both of which are a bit more complicated

Topic: cherry-pick-relative
Reviewers: jerry,brian-k

[aaron-skydio]

Reviews in this chain:
└#121 Cherry-pick can fetch base-branch

[aaron-skydio]

#	head	base	diff	date	summary
0	e813ff2a	62448640	diff	Jul 24 15:26 PM	1 file changed, 17 insertions(+), 6 deletions(-)
1	e7f59d0e	62448640	diff	Jul 24 15:31 PM	1 file changed, 2 insertions(+), 2 deletions(-)

[jerry-skydio]

whoops i put this review off for way too long