bug with .gitattributes diff and embedded NUL

[Eric Blake <ebb9@byu.net>]

The man page for gitattributes suggests that one use of the diff attribute
is to force text-based diffs, even when the file contains embedded NULs.
Yet actually trying this falls flat, since git generates invalid diffs
when encountering embedded NUL.

Here is a pared-down example showing the bug.  I based it off of what I
was trying to do in the git repository for GNU m4, where I was working on
adding testcases to make m4 seamlessly handle embedded NUL.

$ git --version
git version 1.5.4.rc0.67.gf9c5
$ mkdir sample
$ cd sample
$ git init
Initialized empty Git repository in .git/
$ printf 'context with\0embedded nul\nbefore\nmore context\n' > file
$ git add file
$ git commit -m initial
Created initial commit 25108f6: initial
 1 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 file
$ git tag t1
$ sed -i 's/before/after/' file
$ git add file
$ git commit -m later
Created commit a5ee10c: later
 1 files changed, 0 insertions(+), 0 deletions(-)
$ git tag t2
$ git reset --hard t1
HEAD is now at 25108f6... initial
$ sed -i 's/more/some &/' file
$ echo 'file diff' > .gitattributes
$ git add .gitattributes file
$ git commit -m 'switch file to text mode'
Created commit 0f2a9b2: switch file to text mode
 2 files changed, 2 insertions(+), 1 deletions(-)
 create mode 100644 .gitattributes
$ git tag t3
$ git reset --hard t2
HEAD is now at a5ee10c... later
$ git rebase --onto t3 t1
First, rewinding head to replay your work on top of it...
HEAD is now at 0f2a9b2... switch file to text mode
Applying later
error: patch failed: file:1
error: file: patch does not apply
Using index info to reconstruct a base tree...
error: patch failed: file:1
error: file: patch does not apply
Did you hand edit your patch?
It does not apply to blobs recorded in its index.
Cannot fall back to three-way merge.
Patch failed at 0001.

When you have resolved this problem run "git rebase --continue".
If you would prefer to skip this patch, instead run "git rebase --skip".
To restore the original branch and stop rebasing run "git rebase --abort".

Looking closer at .dotest/0001, the diff is indeed invalid, containing the
single line

 context with-before

instead of the desired two lines

 context with^@embedded nul
-before

I'm also wondering if using GIT_EXTERNAL_DIFF would provide a way to work
around this bug in git diff, since GNU diffutils doesn't seem to have any
problems when doing text-mode diffs with embedded NUL:

$ diff -u -a <(git show t1:file) <(git show t2:file) | cat -v
--- /dev/fd/63	2006-11-30 17:00:00.000000000 -0700
+++ /dev/fd/62	2006-11-30 17:00:00.000000000 -0700
@@ -1,3 +1,3 @@
 context with^@embedded nul
-before
+after
 more context

-- 
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net