gitnodekw: do not crash on hg incoming when log template includes gitnode()
Disclaimer: this is my first PR with Heptapod Evolve-based workflow. I am not really sure of what I am doing.
Before this change, if the user's log template included a call to gitnode() to show the git commit hash, invoking "hg incoming" on a git repository that had at least one incoming change failed with:
AttributeError: 'overlayrepo' object has no attribute 'githandler'
Since an incoming changeset should already have well defined git commit hashes, there is no apparent reason for which "hg incoming" should not have this information already.
This change is a workaround that removes any reference to the hg-git structures from the incoming changeset, thus merely avoiding the crash.