Fix configuration syntax to specify customized hunk header patterns.

Fix configuration syntax to specify customized hunk header patterns.

This updates the hunk header customization syntax.  The special
case 'funcname' attribute is gone.

You assign the name of the type of contents to path's "diff"
attribute as a string value in .gitattributes like this:

	*.java diff=java
	*.perl diff=perl
	*.doc diff=doc

If you supply "diff.<name>.funcname" variable via the
configuration mechanism (e.g. in $HOME/.gitconfig), the value is
used as the regexp set to find the line to use for the hunk
header (the variable is called "funcname" because such a line
typically is the one that has the name of the function in
programming language source text).

If there is no such configuration, built-in default is used, if
any.  Currently there are two default patterns: default and java.

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