tests: remove undefined (empty) $opt from test-extdiff.t
It's been undefined for a long time (at least since .t unification).
Differential Revision: https://phab.mercurial-scm.org/D9621
--- a/tests/test-extdiff.t Tue Apr 21 15:13:13 2020 +0200
+++ b/tests/test-extdiff.t Wed Dec 16 21:40:27 2020 -0800
@@ -11,7 +11,7 @@
Should diff cloned directories:
- $ hg extdiff -o -r $opt
+ $ hg extdiff -o -r
Only in a: a
Only in a: b
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