cmd/go: make vcstest repo scripts compatible with old Git versions
The scripts added in CL 421455 passed on the TryBots, but failed on a
subset of the builders that have older 'git' binaries installed.
Notably, the older versions of git do not support:
- 'git branch -m' before the current branch has a commit
- 'init.defaultBranch' in the '.gitconfig' file, and
- 'git branch -c'.
We address those by, respectively:
- waiting to run 'git branch -m' until after the first commit
- always running 'git branch -m' explicitly to set the branch name, and
- using 'git checkout' instead of 'git branch -c' to set branch parents.