Auto merge of #2696 - sbeckeriv:an-author-by-another-name-1213, r=alexcrichton

Auto merge of #2696 - sbeckeriv:an-author-by-another-name-1213, r=alexcrichton

An author by another name

Dearest Reviewer

This is to support environment variables for name and email more like
git. This closes #1213 . I did look in to using libgit2 but I did not
see a clear way to get the author ident. This just moves from or_else
chaining to map_filter with env checks and getting the nth(0) result.

The docs I found
https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables#Committing
do not really indicate an order of preference to the variables. I am
more then happy to rearrange the arrays in anyway.

I also did not add in a new test for the positive case because the
test for the current variables cover that path.

Thanks for reviewing,
Becker