get_diff_hg: Run diff from the file's parent directory

Mercurial's detection for where a file's containing repo is doesn't work
well when the working directory is a different Mercurial repo (c.f.,
mhinz/vim-signify#146).  Run "hg diff" from the target file's parent
directory to work around this.

Signed-off-by: James McCoy <vega.james@gmail.com>