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+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.
+
+The following desription uses "old" and "new" to mean those
+compared entities.
+
+For files in old but not in new (i.e. removed):
+
+    -<mode> \t <type> \t <object> \t <path>
+
+For files not in old but in new (i.e. added):
+
+    +<mode> \t <type> \t <object> \t <path>
+
+For files that differ:
+
+    *<old-mode>-><new-mode> \t <type> \t <old-sha1>-><new-sha1> \t <path>
+
+<new-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 blob    5be4a4.......->000000.......      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-tree-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>.