Use the SDKVersion and not the ProductVersion.

derp.
Add the boxen homebrew bin path so that libssh2 can build via Xcode.
Merge pull request #282 from libgit2/fix-ios-build

Fix Objective-Git iOS build
++xcconfigs to enable assertions in Release builds
Update to latest libgit2.
Massage the diff/patch API in place.
Add some missing diff find flags.
Fix a comment and add documentation.
Add diff merging method.
Cache oldFile and newFile.
Cache lines in GTDiffHunk.
Cache GTDiffFile's OID.
Document.
Add git_patch_size wrapper.
Cache hunks in GTPatch.
Tests++;
Stray semicolon.
content => contents.
Provide helpful `-debugDescription`s for those.
Don't use a temporary variable; it's cached.