merge: provide *_ISLINK environment vars to merge helper

Sets HG_MY_ISLINK, HG_OTHER_ISLINK, HG_BASE_ISLINK in environment. Without these variables, it's impossible for the merge application to know whether the 'other' and 'base' files were symlinks in their original contexts. For the purposes of the merge they are always emitted as small text files.

mercurial/merge.py

@@ -53,7 +53,10 b' def filemerge(repo, fw, fd, fo, wctx, mc'
     r = util.system('%s "%s" "%s" "%s"' % (cmd, a, b, c), cwd=repo.root,
                     environ={'HG_FILE': fd,
                              'HG_MY_NODE': str(wctx.parents()[0]),
-                             'HG_OTHER_NODE': str(mctx)})
+                             'HG_OTHER_NODE': str(mctx),
+                             'HG_MY_ISLINK': fcm.islink(),
+                             'HG_OTHER_ISLINK': fco.islink(),
+                             'HG_BASE_ISLINK': fca.islink(),})
     if r:
         repo.ui.warn(_("merging %s failed!\n") % fd)