Add GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH

This adds an option to checkout a la the diff option to turn off
fnmatch evaluation for pathspec entries.  This can be useful to
make sure your "pattern" in really interpretted as an exact file
match only.
Merge pull request #1221 from arrbee/checkout-without-pathspec-match

Add GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH