Removed unnecessary changes to Tests project.
MergeFixture: Cleaned usings & removed unused method.
NativeMethods: Used suggested signature for git_merge.
Added ref+fetchhead git_merge_froms just in case they could be used in
future (Should I revert this, I'm not sure on your rules about unused
methods in a wrapper?)
Proxy: Split git_merge and git_merge_from_oid into separate methods and
used suggested signature. Fixed the result checking for boolean C calls
in is_uptodate and is_fastforward.
GitMergeResult: Renamed to MergeResult, put loading prop values in
constructor.
Repository: Replaced MergeOnto with Merge.
MergeFixture: Added in recommended asserts
MergeResult: Fixed up bad reference + added FastForwardOid prop
Repository: Wrapped MergeHead and MergeResult handles in usings.