Xcode wart.

This was duplicated in the `tree-contents` pull request I made some time ago.
Renames for consistency

`sha` => `SHA` and `oid` => `OID`, with an exception on the latter when the parameter is a `git_oid` (like the `GTOID` initializer).
Merge pull request #216 from tiennou/xcode-wart

Xcode wart.
Merge pull request #217 from tiennou/rename-sha-oid

Renames for consistency
Fix missed rename.