FastForward merge should block if there are checkout conflicts.

Currently, if there are changes in your working directory and
a fastforward merge would update those files, the checkout
that is performed as part of the fast forward will skip over
these files. This change fixes and adds regression tests around
this behavior.
Update binaries to 091165c

https://github.com/libgit2/libgit2/compare/e93206e...091165c
Fix for osx CI script
Index.Stage: do not stage ignored files

Stage() now respects ignore files by default, similar to git.git.
Users accustomed to the old behavior may use the new StageOptions
class (in particular the IncludeIgnored bool) to override this
behavior.