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

@@ -1,26 +1,26 b''
 #!/bin/sh
 
 HGRCPATH=$HGTMP/.hgrc; export HGRCPATH
 echo "[extensions]" >> $HGTMP/.hgrc
 echo "extdiff=" >> $HGTMP/.hgrc
 
 hg init a
 cd a
 echo a > a
 hg add
 hg extdiff -o -Nr
 
 echo "[extdiff]" >> $HGTMP/.hgrc
 echo "cmd.falabala=echo" >> $HGTMP/.hgrc
 echo "opts.falabala=diffing" >> $HGTMP/.hgrc
 
 hg falabala
 
 hg help falabala
 
 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

@@ -1,31 +1,32 b''
 adding a
 making snapshot of 0 files from rev 000000000000
 making snapshot of 1 files from working dir
 diff -Nr a.000000000000/a a/a
 0a1
 > a
 making snapshot of 0 files from rev 000000000000
 making snapshot of 1 files from working dir
 diffing a.000000000000 a
 hg falabala [OPT]... [FILE]...
 
 use 'echo' to diff repository (or selected files)
 
             Show differences between revisions for the specified
             files, using the 'echo' program.
 
             When two revision arguments are given, then changes are
             shown between those revisions. If only one revision is
             specified then that revision is compared to the working
             directory, and, when no revisions are specified, the
             working directory files are compared to its parent.
 
 options:
 
  -o --option   pass option to comparison program
  -r --rev      revision
  -I --include  include names matching the given patterns
  -X --exclude  exclude names matching the given patterns
 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