[PATCH] Split external diff command interface to a separate file.

With this patch, the non-core'ish part of show-diff command that
invokes an external "diff" comand to obtain patches is split
into a separate file.  The next patch will introduce a new
command, diff-tree-helper, which uses this common diff interface
to format diff-tree and diff-cache output into a patch form.

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

diff.h

@@ -0,0 +1,17 @@
+#ifndef DIFF_H
+#define DIFF_H
+
+extern void prepare_diff_cmd(void);
+
+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 */);
+
+extern void show_diff_empty(const unsigned char *sha1,
+			    const char *name,
+			    int reverse);
+
+#endif /* DIFF_H */