Update to libgit2 494be42
Fix interface changes due to libgit2 upgrade

* Most repository changing functions now need a signature
* Duplication functions i.e. git_signature_dup now return an error code
* libgit2's merge API has renamed some constants/options/flags
travis: install Pod::Coverage::TrustPod as well
MakeMaker.pm: fix regex include path on Win32
Changes: update