ignore: consider files with a CR in their names

We currently consider CR to start the end of the line, but that means
that we miss cases with CR CR LF which can be used with git to match
files whose names have CR at the end of their names.

The fix from the patch comes from Russell's comment in the issue.

This fixes #2536.
Merge pull request #2688 from libgit2/cmn/ignore-file-trailing-cr

ignore: consider files with a CR in their names