mq: extend support for the --mq argument to extension commands
This allows commands like `purge' to accept the --mq option.
$ hg init
$ touch a
$ hg add a
$ hg ci -m "a" -d "1000000 0"
$ echo 123 > b
$ hg add b
$ hg diff --nodates
diff -r acd8075edac9 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
$ hg diff --nodates -r tip
diff -r acd8075edac9 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
$ echo foo > a
$ hg diff --nodates
diff -r acd8075edac9 a
--- a/a
+++ b/a
@@ -0,0 +1,1 @@
+foo
diff -r acd8075edac9 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
$ hg diff -r ""
abort: 00changelog.i@: ambiguous identifier!
$ hg diff -r tip -r ""
abort: 00changelog.i@: ambiguous identifier!
$ true