diff -r b19d8d5d6b51 -r 61f4d59e9a0b tests/failfilemerge.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/failfilemerge.py	Fri Feb 05 01:56:46 2016 +0000
@@ -0,0 +1,18 @@
+# extension to emulate interupting filemerge._filemerge
+
+from __future__ import absolute_import
+
+from mercurial import (
+    filemerge,
+    extensions,
+    error,
+)
+
+def failfilemerge(filemergefn,
+        premerge, repo, mynode, orig, fcd, fco, fca, labels=None):
+    raise error.Abort("^C")
+    return filemergefn(premerge, repo, mynode, orig, fcd, fco, fca, labels)
+
+def extsetup(ui):
+    extensions.wrapfunction(filemerge, '_filemerge',
+                            failfilemerge)