Update libgit2 binaries to 32e4992

https://github.com/libgit2/libgit2/compare/eb04744...32e4992
Drop dead code

In `git_odb_write()`, libgit2 already calls `git_odb_exists()`
before invoking the backend write() method.
Drop dead code

`FinalizeWrite()` is only invoked if the object pointed at by the oid
isn't found in any backend.
Make OdbBackend expose valid backend return codes
Small refactoring
Add GIT_EUSER to OdbBackend exposed return codes
Slight OdbBackendFixture refactoring
Introduce ObjectId.StartsWith()
Test Push with a custom backend
Release LibGit2Sharp v0.14.0
Drop obsolete members after release v0.14
Normalize submodule paths on lookup

Fixes libgit2/libgit2#1845