Fall back on main / master if one doesn't exist

[jerry-skydio]

It's a very common error to fail to initialize the
main_branch arg for a project, and may be an early
friction point. We can automatically handle this
for most cases by just using the other if one
doesn't exist.

Topic: main_fallback
Reviewers: brian-k

[jerry-skydio]

Reviews in this chain:
└#20 Fall back on main / master if one doesn't exist

[jerry-skydio]

#	head	base	diff	date	summary
0	4e6acf6c	7d99cd87	diff	Sep 07 1:42 PM	1 file changed, 11 insertions(+), 1 deletion(-)
1	efb6e088	7d99cd87	diff	Sep 07 1:45 PM	1 file changed, 1 insertion(+), 1 deletion(-)