Remove pipe-based git command line usage

[rowanj]

GitX should use libgit2 & objective-git instead of running the git command line tools and parsing the output.

The pipe APIs used are deprecated, and they interact poorly with debug tools.

[rowanj]

Xcode 4.3 beta is distributed as a plain "Xcode.app" bundle, so git isn't even accessible from the command line by default with Xcode installed, by the look of it. This bumps the priority of working on this, I'd like to have it done by the time Xcode 4.3 hits general release.

[rowanj]

To clarify the 'why' of this:

• The git binary that is found is not always the same one that the user will get on the command line
• Command responses are parsed from stdout and stderr of the binary. Not all cases are handled, and it is fragile to any message format changes or localizations.
• It introduces a dependancy on an externally-installed program
• Any bugs due to the version/configuration of the user's git program will be difficult to diagnose or reproduce
• We want to be able to run out-of-the-box with current versions of Xcode, and it seems 4.3 is not installing command-line developer tools anymore

[rowanj]

We're going to be reliant on command-line git to some extent at least until libgit2 implements the git@ (ssh) transport.

Still, that shouldn't stop us from converting to libgit2 for local operations at least.