merge: supply base node to merge tools in the environment

Merge tools will be able to exploit this to correctly merge backouts.
This won't work fully, though, until issue 1327 is solved, since the
node information is not necessarily correct.

mercurial/filemerge.py

@@ -186,6 +186,7 b' def filemerge(repo, mynode, orig, fcd, f'
     env = dict(HG_FILE=fd,
                HG_MY_NODE=short(mynode),
                HG_OTHER_NODE=str(fco.changectx()),
+               HG_BASE_NODE=str(fca.changectx()),
                HG_MY_ISLINK='l' in fcd.flags(),
                HG_OTHER_ISLINK='l' in fco.flags(),
                HG_BASE_ISLINK='l' in fca.flags())