Git util failing in non git directory

When running netlify build in a folder that’s not a git repository, the git utility blows up with the message:

Error: Invalid base commit master
Command failed with exit code 128: git rev-parse master
fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
    at findRef (/Users/mbc/.nvm/versions/node/v8.11.3/lib/node_modules/netlify-cli/node_modules/@netlify/git-utils/src/refs.js:43:11)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

Steps to reproduce

run netlify build in a folder that’s not a git repository

Expected behavior

Either ignore the git util or give a clear error message about needing a git repository to run.

[ehmicky]

After discussing with @erquhart we concluded on the following solution: when there is no .git directory, the git utility should be an empty object and not report/print any errors.

This means:

• we do not need to modify the utility’s API
• users without a .git repository that do not use the git utility do not see any errors (i.e. same behavior as without the Build beta)
• users without a .git repository that use the git utility will most likely get an error about the property/method they are trying to use not existing. This is not the best error reporting experience, but we can improve this later.

[ehmicky]

Done at #760.