Submodule libgit2sharp updated

Submodule libgit2sharp:
    > abc7b61 - Fixed a typo.
    > bd736e2 - Clean up cancellation patterns in callbacks and other small cleanups.
    > 981f439 - Expose Push progress reporting
    > 31291fe - Introduce blob FilteringOptions for content stream
    > 4d4ebe2 - Allow push APIs to set packbuilder parallelism
    > e2290f8 - Enforce repo.Network.Push()  test coverage
    > b10c4b6 - Make string marshaling either lax or strict
    > 75eaceb - Introduce EncodingMarshaler.Cleanup()
    > 11f4cb8 - Split Patch and TreeChanges generation
    > c5419f7 - Refactor Marshalers
    > 277ae16 - Protect TagAnnotation creation from invalid parameters
    > 28d1820 - Protect TagAnnotation creation from name or message containing '\0'
    > c8ba327 - Protect Commit creation from message containing '\0'
    > 0d713cb - Protect Signature creation from invalid parameters
    > 374f2a5 - Protect Signature creation from components containing '\0'
    > 8528d4c - Enforce creation of unborn branch test coverage