tree: accept null ids in existing trees when updating

Pack file verification

Coverity flavored clang analyzer fixes

worktree: unlock should return 1 when the worktree isn't locked

Fix 'invalid packet line' for ng packets containing errors

Otherwise we return a NULL context, which will get dereferenced in 
apply_credentials.

Otherwise we'll return stack data to the caller.

In case there was nothing to parse in the buf, we'd return uninitialized
stack data.

At line 594, we do this :
if (error < 0)
		return error;

but if nothing was pushed in a GIT_SORT_TIME revwalk, we'd return
uninitialized stack data.