[PATCH] Diff-tree-helper take two.

This reworks the diff-tree-helper and show-diff to further make external
diff command interface simpler.

These commands now honor GIT_EXTERNAL_DIFF environment variable which
can point at an arbitrary program that takes 7 parameters:

  name file1 file1-sha1 file1-mode file2 file2-sha1 file2-mode

The parameters for an external diff command are as follows:

  name        this invocation of the command is to emit diff
	      for the named cache/tree entry.

  file1       pathname that holds the contents of the first
	      file.  This can be a file inside the working
	      tree, or a temporary file created from the blob
	      object, or /dev/null.  The command should not
	      attempt to unlink it -- the temporary is
	      unlinked by the caller.

  file1-sha1  sha1 hash if file1 is a blob object, or "."
	      otherwise.

  file1-mode  mode bits for file1, or "." for a deleted file.

If GIT_EXTERNAL_DIFF environment variable is not set, the
default is to invoke diff with the set of parameters old
show-diff used to use.  This built-in implementation honors the
GIT_DIFF_CMD and GIT_DIFF_OPTS environment variables as before.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff.h

@@ -1,17 +1,31 @@
+/*
+ * Copyright (C) 2005 Junio C Hamano
+ */
 #ifndef DIFF_H
 #define DIFF_H
 
-extern void prepare_diff_cmd(void);
+/* These two are for backward compatibility with show-diff;
+ * new users should not use them.
+ */
+extern void show_differences(const struct cache_entry *ce, int reverse);
+extern void show_diff_empty(const struct cache_entry *ce, int reverse);
 
-extern void show_differences(const char *name, /* filename on the filesystem */
-			     const char *label, /* diff label to use */
-			     void *old_contents, /* contents in core */
-			     unsigned long long old_size, /* size in core */
-			     int reverse /* 0: diff core file
-					    1: diff file core */);
+struct diff_spec {
+	union {
+		const char *name;       /* path on the filesystem */
+		unsigned char sha1[20]; /* blob object ID */
+	} u;
+	unsigned short mode;	 /* file mode */
+	unsigned sha1_valid : 1; /* if true, use u.sha1 and trust mode.
+				  * (however with a NULL SHA1, read them
+				  * from the file!).
+				  * if false, use u.name and read mode from
+				  * the filesystem.
+				  */
+	unsigned file_valid : 1; /* if false the file does not even exist */
+};
 
-extern void show_diff_empty(const unsigned char *sha1,
-			    const char *name,
-			    int reverse);
+extern void run_external_diff(const char *name,
+			      struct diff_spec *, struct diff_spec *);
 
 #endif /* DIFF_H */