filemerge: add config item for GUI tools

<tool>.gui indicates whether a tool requires a GUI to run

mercurial/filemerge.py

@@ -36,6 +36,8 b' def _picktool(repo, ui, path, binary, sy'
             ui.warn(_("tool %s can't handle symlinks\n") % tmsg)
         elif binary and not _toolbool(ui, tool, "binary"):
             ui.warn(_("tool %s can't handle binary\n") % tmsg)
+        elif not util.gui() and _toolbool(ui, tool, "gui"):
+            ui.warn(_("tool %s requires a GUI\n") % tmsg)
         else:
             return True
         return False

mercurial/util.py

@@ -895,6 +895,10 b' def splitpath(path):'
     function if need.'''
     return path.split(os.sep)
 
+def gui():
+    '''Are we running in a GUI?'''
+    return os.name == "nt" or os.name == "mac" or os.environ.get("DISPLAY")
+
 # Platform specific variants
 if os.name == 'nt':
     import msvcrt