git: how to produce context diffs?

[Bruno Haible <bruno@clisp.org>]

Hi,

Is this a bug in git-diff? The git-diff-files.html says:

  " When the environment variable GIT_EXTERNAL_DIFF is not set ...
    For example, if you prefer context diff:
    GIT_DIFF_OPTS=-c git-diff-index -p HEAD  "

This doesn't work for me with git-1.4.4:

$ unset GIT_EXTERNAL_DIFF
$ export GIT_DIFF_OPTS=-c
$ git-diff-index -p HEAD
diff --git a/configure.ac b/configure.ac
index 74901dc..d222ded 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,5 +1,5 @@
 dnl Process this file with autoconf to produce a configure script.
-AC_INIT(hello, 2.1.1, bug-gnu-hello@gnu.org)
+AC_INIT(hello, 2.1.2, bug-gnu-hello@gnu.org)
 AC_CONFIG_SRCDIR([src/hello.c])
 
 AC_PREREQ(2.52)

Expected output:

$ git-diff-index -p HEAD
index 74901dc..d222ded 100644
diff --git a/configure.ac b/configure.ac
*** a/configure.ac
--- b/configure.ac
***************
*** 1,5 ****
  dnl Process this file with autoconf to produce a configure script.
! AC_INIT(hello, 2.1.1, bug-gnu-hello@gnu.org)
  AC_CONFIG_SRCDIR([src/hello.c])
  
  AC_PREREQ(2.52)
--- 1,5 ----
  dnl Process this file with autoconf to produce a configure script.
! AC_INIT(hello, 2.1.2, bug-gnu-hello@gnu.org)
  AC_CONFIG_SRCDIR([src/hello.c])
  
  AC_PREREQ(2.52)


(Really, while I find -u diffs fine for tiny changes, I find them unreadable
for rewrites of larger blocks, and cannot live without -c for these.)

When I look at diff.c around
       const char *diffopts = getenv("GIT_DIFF_OPTS");
it appears that only unified diffs are supported??