bin/git-land: change default base branch name
bin/git-land: change default base branch name

We're switching our default branch name from "master" to
"main". This will undoubtedly require many small tweaks
in various places; this is one.

Closes chain/chainprv#609.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@bobg

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain