Initial stab at wrapping `git_index_update_all`
Still to be done: complete tests.
Clarify documentation
Use proper casting (when passing in the block) to prevent mangling memory
Continue work on test
The only thing now left is how to get a test repo added, which I have no idea how to do.
Continue work on the `updateEntireIndex` test

Unfortunately this only works locally
Whitespace—
Fix spacing issues
Documentation Fixes
Rewrite test to use fixtures.
Because the test now uses fixtures, we can remove unneeded code.
Delete space
If the block is nil then we shouldn't pass in the callback
Update GTRepositoryStatusSpec to use `updateEntireIndex:` instead of  `git_index_update_all`
Add space back in
Document that all parameters accept nil
Change method signature from `updateEntireIndex:` to `updatePathspecs:`

While we’re at it:
- Change the ordering of the method parameters and block.
- Change block type to BOOL.
- Add a `BOOL *stop` parameter to the block.
Block cannot be nil.
Update documentation
Remove extraneous whitespace.
When setting `stop` to `YES` and returning `YES` abort after the file has been added.
Style fix.