Copier fails if git is not installed

[yajo]

Executive summary
Currently copier won't work if git is not installed, even if you just want to use it to render and copy files from some local folder to another, where git is never involved in the process.

Would it be possible to remove git-dependence?

Details
Copier wants to find out whether the source directory has anything to do with git, and calls the git-executable to check this. But if it is not found then it fails.
My suggestion is to make a little change: when copier is unable to find git-executable then simply assume that source is not under git and proceed.
I tried this approach (modified is_git_bundle to return False if it fails to find git) and it works perfectly.

Originally posted by @kefirbandi in #1634