Update appcast script for python3
Latest objective-git
Latest objective-git: API changes

- [GTRepo lookupObjectByRevParse:error:] -> - [GTRepo lookUpObjectByRevParse:error:]
Sidestep error due to wrong class in the submodules collection

Remove PBGitSubmodule for now; just use GTSubmodule.
Merge remote-tracking branch 'refs/remotes/origin/master'
Fix work dir discovery for latest objective-git API
Fix segfault when forcing commit after a hook fails
Add requirements for build script
Fix import case in appcast.py
Fix appcast.py script under Python 3 and update the template
Late updates to the change log for 0.15.1869
Update README
Don't pass down debug flags to CLI git.
Warn: Duplicate assignment.
Move Foundation.framework to frameworks group.
Remove unused constructors and commented-out code from PBGitLane
Remove the PBGitSHA class and use the GTOID class from Objective-Git instead.
Remove unnecessary copying/passing by value of git_oid
As per unmaintained notice on Sparkle, switch to pornel's fork.
Let's run the analyser when building.