Log Message:
Update ruby-rugged to 0.26.0.

* Allow disabling rename detection for merge operations.
* Documentation fix: Iterator -> Enumerator
* Use Gem.win_platform? instead of custom code
* add a little hint in the readme for google
* cmake: honor CMAKE_FLAGS environment variable
* cmake: honor `--with-sha1dc`
* Better error messages for bad signature hash keys
* Catch up to libgit2 0.26.
* Add a method to parse a signature from a buffer