Re-add the interactive merge

Re-add the interactive merge

This feature was dropped by previous changes to
git.merge_recursive(). This patch modifies gitmergeonefile.merge() to
only deal with interactive merges or simply check out the conflict
stages. The stgit.commands.common.resolved() function was moved to
git.resolved(). The patch also drops the git.merge() function since it
can no longer use gitmergeonefile.merge() (different API) and modifies
the 'sync' command to always use git.merge_recursive().

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>