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
regex: Fixed several warnings about signed/unsigned conversions.
regex: Fixed warnings about unused parameter values.

There are different solutions to the problem. In this change, we
define an UNUSED macro that maps to __attribute__((unused)) when
compiling with gcc. Otherwise it is a NOOP. We apply this macro
in all function headers for each parameter value that is not used
within the function body.

The change is local to regex.