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.

Updates #27494.

Change-Id: I42f012f5add8f31e41d077d752d8268aacbce8a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/445355
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

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