- doing mkdir before `git init <dir>` is not necessary; git will
create the directory if it doesn't exist
- passing both `-C` and `--git-dir` is not needed for git invocations
- reading the current working directory isn't necessary for specifying
nested paths to operate in
- use `git add .` instead of explicitly naming all paths; we generate a
gitignore for binaries that we want to avoid adding
- generate cross-platform gitignore for Go binary extensions
- add helpers to simplify writing files, tests
a65b9a3831