Add automatic git-mergetool invocation to the new infrastructure

Add automatic git-mergetool invocation to the new infrastructure

This patch adds the IndexAndWorktree.mergetool() function responsible
for calling 'git mergetool' to interactively solve conflicts. The
function may also be called from IndexAndWorktree.merge() if the
standard 'git merge-recursive' fails and 'interactive == True'. The
'allow_interactive' parameter is passed to Transaction.push_patch() from
the functions allowing interactive merging.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
Acked-by: Karl Hasselström <kha@treskal.com>