Why 'svn diff --diff-cmd=diff ' output inner diff format?

Question

I found diff output is strange when I set --diff-cmd=diff.

➜  svntest  svn diff --diff-cmd=diff -x '' #The cmd `diff` cann't output this format, so strange 
Index: a.c
===================================================================
--- a.c (revision 1)
+++ a.c (working copy)
@@ -0,0 +1 @@
+teste

➜  svntest  svn diff --diff-cmd=diff -x '-i'
Index: a.c
===================================================================
0a1
> teste

I thought the two commands above essentially excute as below, am I wrong?

➜  svntest  diff   -L 'a.c(revision 1)' -L 'a.c(working copy)' '/Users/hilojack/www/svntest/.svn/pristine/da/da39a3ee5e6b4b0d3255bfef95601890afd80709.svn-base' '/Users/hilojack/www/svntest/a.c'
0a1
> teste
➜  svntest  diff  -i -L 'a.c(revision 1)' -L 'a.c(working copy)' '/Users/hilojack/www/svntest/.svn/pristine/da/da39a3ee5e6b4b0d3255bfef95601890afd80709.svn-base' '/Users/hilojack/www/svntest/a.c'
0a1
> teste

---

I get this from svn help diff

-x [--extensions] ARG    : Default: '-u'. When Subversion is invoking an external diff program, ARG is simply passed along to the program.

The subversion will pass default params -u to external diff program.

➜  svntest  svn diff --diff-cmd=echo
Index: a.c
===================================================================
-u -L a.c   (revision 1) -L a.c (working copy) /Users/hilojack/www/svntest/.svn/pristine/da/da39a3ee5e6b4b0d3255bfef95601890afd80709.svn-base /Users/hilojack/www/svntest/a.c

Answer 1

Subversion passes the following parameters to the external diff command:

• -u or the user specified flags via -x'. If-xis null, the-u` is passed anyway.
• -L
• Base Title
• -L
• Working Copy Title
• Base file
• Working copy file

The only way to get rid of -u is to pass in another parameter. I wrote a Perl script I use to do my parsing and then use VIM for my diff:

#! /usr/bin/env perl

use strict;
use warnings;

use constant DIFF => qw(mvim -d -f);

my $parameters = $#ARGV;
my $file1 = $ARGV[$parameters - 1];
my $file2 = $ARGV[$parameters];
my $title1 = $ARGV[$parameters - 4];
my $title2 = $ARGV[$parameters - 2];

$ENV{TITLE} = "$title1  -   $title2";
system DIFF, '-c', 'let &titlestring=$TITLE', $file1, $file2;