Don't require a diff which accepts the -N option.

tests/test-extdiff

@@ -1,38 +1,34 b''
 #!/bin/sh
 
 echo "[extensions]" >> $HGRCPATH
 echo "extdiff=" >> $HGRCPATH
 
 hg init a
 cd a
 echo a > a
 hg add
-diff -N /dev/null /dev/null 2> /dev/null
-if [ $? -ne 0 ]; then
-	opt="-p gdiff"
-fi
-hg extdiff -o -Nr $opt
+hg extdiff -o -r $opt
 
 echo "[extdiff]" >> $HGRCPATH
 echo "cmd.falabala=echo" >> $HGRCPATH
 echo "opts.falabala=diffing" >> $HGRCPATH
 
 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
 
 # test diff during merge
 hg update 0
 echo b >> b
 hg add b
 hg ci -m "new branch" -d '1 0'
 hg update -C 1
 hg merge tip
 hg falabala || echo "diff-like tools yield a non-zero exit code"

tests/test-extdiff.out

@@ -1,39 +1,37 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
+Only in a: 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
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 1 files updated, 0 files merged, 1 files removed, 0 files unresolved
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 (branch merge, don't forget to commit)
 making snapshot of 1 files from rev 5e49ec8d3f05
 making snapshot of 1 files from working dir
 diffing a.5e49ec8d3f05 a
 diff-like tools yield a non-zero exit code