Deduce base fallback from repo's branches

[ErichDonGubler]

Right now, git-glimpse determines a base value for subcommands like stack using the following steps:

1. If --base was specified (is Some), then use that.
2. Consult glimpse.base in Git configuration. If set, then use that.
3. Otherwise, use main.

We should add a step between (2) and (3) where we attempt to deduce a mainline branch. Shelling out to git br -l main origin/main master origin/master is likely to be useful, since it will only output the branches that exist in the order specified.