Per-path attribute based hunk header selection.

This makes"diff -p" hunk headers customizable via gitattributes mechanism.
It is based on Johannes's earlier patch that allowed to define a single
regexp to be used for everything.

The mechanism to arrive at the regexp that is used to define hunk header
is the same as other use of gitattributes.  You assign an attribute, funcname
(because "diff -p" typically uses the name of the function the patch is about
as the hunk header), a simple string value.  This can be one of the names of
built-in pattern (currently, "java" is defined) or a custom pattern name, to
be looked up from the configuration file.

  (in .gitattributes)
  *.java   funcname=java
  *.perl   funcname=perl

  (in .git/config)
  [funcname]
    java = ... # ugly and complicated regexp to override the built-in one.
    perl = ... # another ugly and complicated regexp to define a new one.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore.h

@@ -27,6 +27,7 @@ struct diff_filespec {
 	char *path;
 	void *data;
 	void *cnt_data;
+	const void *hunk_header_ident;
 	unsigned long size;
 	int xfrm_flags;		 /* for use by the xfrm */
 	unsigned short mode;	 /* file mode */