Fixed test-extdiff.

diff-like tools usually return 1 if differences are found.
Currently the extdiff returns 256, but that's a different topic
(consolidating return code handling in hg).

tests/test-extdiff

@@ -23,4 +23,4 b" hg ci -d '0 0' -mtest1"
 echo b >> a
 hg ci -d '1 0' -mtest2
 
-hg falabala -r 0:1
+hg falabala -r 0:1 || echo "diff-like tools yield a non-zero exit code"

tests/test-extdiff.out

@@ -29,3 +29,4 b' options:'
 making snapshot of 1 files from rev e27a2475d60a
 making snapshot of 1 files from rev 5e49ec8d3f05
 diffing a.e27a2475d60a a.5e49ec8d3f05
+diff-like tools yield a non-zero exit code