vcs-jump: fix Mercurial merge invocation from subdirectory
vcs-jump: fix Mercurial merge invocation from subdirectory

Actually tested it now: it works, although it is probably ridiculously
fragile.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent