Dramatically reduce command-line `git` parsing when loading rev list

Issue #2
Populate PBGitCommit dynamically
Looks like --left-right wasn't used

Exception may be if passed from the command line, but for now it's
just complicating things.
Prototype loading commits without command-line
Load tag refs, too
Redistribute work of loading commits
Add a reference to the libgit2 source to the Xcode project
Continue cleaning up PBGitRevList
Filter out changes to lock files earlier in the file watcher callback chain
The answer to "is the index changed" is easier for bare repositories
Cache commits within the rev list

They're immutable anyway
Use Objective-Git to create tags

Issue #2