Update to upstream libgit2
++libgit2 to latest development
Update GTObjectDatabase for libgit2 changes
++libgit2 to is-empty-fix to fix unit test failures
++libgit2
Merge pull request #244 from libgit2/update-libgit2

Update libgit2 to latest development
Add an OID property to GTDiffFile.
Minor cleanup
Merge pull request #249 from libgit2/gtdifffile-oid

Add an OID property to GTDiffFile
Convert -(NSString *)message, -(NSString *)name, -(GTObject *)target, and -(NSString *)targetType to properties.
Compile libgit2 with libssh2 for iOS
Use git directories URL for equality.
Use git directory here.
Fix diff spec.
Update GTTag properties to use modern objective-c syntax.
Add options argument to GTRepository cloneFromURL
Add GTTagSpec and delete GTTagTest.
(The previous commit accidentally made those changes, this just updates the Project Settings)
++libgit2
Merge pull request #251 from libgit2/testrepo-bare

Make testrepo.git really bare.
Make new properties readonly.

(Style fixes as well)