doctr-versions-menu CLI fails when run outside of git repo

[goerz]

The doctr-versions-menu command runs "git add" for several of the files it creates in the current working directory (e.g. index.html). This makes sense for the tool's original intent to run in the context of doctr, i.e. in the root of a gh-pages branch.

However, when deploying documentation not via gh-pages, but to a regular static website host, the current working directory (the server root) will usually not be a git repository. Attempting to run git add will fail and crash doctr-versions-menu.

For now, it should be fine to just silently ignore failures of git add.

In the long term, we should add a --git/--no-git options. If --git is given, doctr-versions-menu will run git add for any created files and return an error if this fails (the current behavior). If --no-git is given, doctr-versions-menu will not run git add at all. If neither options is given, doctr-versions-menu will try to run git add, but fail silently.

Reported by @ablech