merge: print status message before launching external merge tool

It seems somewhat common that people run into a merge conflict and
don't notice the launched merge tool, and instead they think hg just
hung. Let's print a message for each file that we launch a GUI merge
tool for.

mercurial/filemerge.py

@@ -489,6 +489,9 b' def _xmerge(repo, mynode, orig, fcd, fco'
     args = util.interpolate(r'\$', replace, args,
                             lambda s: util.shellquote(util.localpath(s)))
     cmd = toolpath + ' ' + args
+    if _toolbool(ui, tool, "gui"):
+        repo.ui.status(_('running merge tool %s for file %s\n') %
+                       (tool, fcd.path()))
     repo.ui.debug('launching merge tool: %s\n' % cmd)
     r = ui.system(cmd, cwd=repo.root, environ=env)
     repo.ui.debug('merge tool returned: %s\n' % r)