Add option to avoid doing a git checkout when using the exec template function

The exec template function is used to run a command in a checkout of a git/hg repository. However in some cases we use the exec function to run git commands where it doesn't matter what is the current commit in the working tree. This is the case for example in the abbrev config option which we use in tor-browser-build in a lot of filenames.

I think it would be possible to improve performance by avoiding some git checkout calls with this option.