filemerge: do what the context __bytes__ does, but locally

str() here is clearly the wrong thing, and I think the code is clearer
when it doesn't just depend on the magic __{str,bytes}__ behavior.

I decided to grep around for \sstr\( and see what low-hanging fruit
that showed me. This was part of that hunt. That grep pattern still
has some things worth exploring.

Differential Revision: https://phab.mercurial-scm.org/D2442

mercurial/filemerge.py

@@ -520,8 +520,8 def _xmerge(repo, mynode, orig, fcd, fco
             baselabel = 'base'
         env = {'HG_FILE': fcd.path(),
                'HG_MY_NODE': short(mynode),
-               'HG_OTHER_NODE': str(fco.changectx()),
-               'HG_BASE_NODE': str(fca.changectx()),
+               'HG_OTHER_NODE': short(fco.changectx().node()),
+               'HG_BASE_NODE': short(fca.changectx().node()),
                'HG_MY_ISLINK': 'l' in fcd.flags(),
                'HG_OTHER_ISLINK': 'l' in fco.flags(),
                'HG_BASE_ISLINK': 'l' in fca.flags(),