The output format from "git-diff-cache", "git-diff-tree" and
"git-diff-files" is very similar.

These commands all compare two sets of things; what are
compared are different:

git-diff-cache <tree-ish>::
        compares the <tree-ish> and the files on the filesystem.

git-diff-cache --cached <tree-ish>::
        compares the <tree-ish> and the cache.

git-diff-tree [-r] <tree-ish-1> <tree-ish-2> [<pattern>...]::
        compares the trees named by the two arguments.

git-diff-files [<pattern>...]::
        compares the cache and the files on the filesystem.


An output line is formatted this way:

  ':' <mode> ' ' <mode> ' ' <sha1> ' ' <sha1> I <path> I <path> L

By default, I and L are '\t' and '\n' respectively.  When '-z'
flag is in effect, both I and L are '\0'.

In each <mode>, <sha1> and <path> pair, left hand side describes
the left hand side of what is being compared (<tree-ish> in
git-diff-cache, <tree-ish-1> in git-diff-tree, cache contents in
git-diff-files).  Non-existence is shown by having 000000 in the
<mode> column.  That is, 000000 appears as the first <mode> for
newly created files, and as the second <mode> for deleted files.

Usually two <path> are the same.  When rename/copy detection is
used, however, an "create" and another "delete" records can be
merged into a single record that has two <path>, old name and
new name.

<sha1> is shown as all 0's if new is a file on the filesystem
and it is out of sync with the cache.  Example:

  :100644 100644 5be4a4...... 000000......    file.c    file.c


Generating patches with -p
--------------------------

When "git-diff-cache", "git-diff-tree", or "git-diff-files" are run
with a '-p' option, they do not produce the output described above
instead they produce a patch file.

The patch generation can be customized at two levels.  This
customization also applies to "git-diff-helper".

1. When the environment variable 'GIT_EXTERNAL_DIFF' is not set,
   these commands internally invoke "diff" like this:

      diff -L a/<path> -L a/<path> -pu <old> <new>
+
For added files, `/dev/null` is used for <old>.  For removed
files, `/dev/null` is used for <new>
+
The "diff" formatting options can be customized via the
environment variable 'GIT_DIFF_OPTS'.  For example, if you
prefer context diff:

      GIT_DIFF_OPTS=-c git-diff-cache -p $(cat .git/HEAD)


2. When the environment variable 'GIT_EXTERNAL_DIFF' is set, the
   program named by it is called, instead of the diff invocation
   described above.
+
For a path that is added, removed, or modified,
'GIT_EXTERNAL_DIFF' is called with 7 parameters:

     path old-file old-hex old-mode new-file new-hex new-mode
+
where:

     <old|new>-file:: are files GIT_EXTERNAL_DIFF can use to read the
		      contents of <old|ne>,
     <old|new>-hex:: are the 40-hexdigit SHA1 hashes,
     <old|new>-mode:: are the octal representation of the file modes.

+ 
The file parameters can point at the user's working file
(e.g. `new-file` in "git-diff-files"), `/dev/null` (e.g. `old-file`
when a new file is added), or a temporary file (e.g. `old-file` in the
cache).  'GIT_EXTERNAL_DIFF' should not worry about unlinking the
temporary file --- it is removed when 'GIT_EXTERNAL_DIFF' exits.

For a path that is unmerged, 'GIT_EXTERNAL_DIFF' is called with 1
parameter, <path>.