userdiff/perl: tighten BEGIN/END block pattern to reject here-doc delimiters

userdiff/perl: tighten BEGIN/END block pattern to reject here-doc delimiters

A naive method of treating BEGIN/END blocks with a brace on the second
line as diff/grep funcname context involves also matching unrelated
lines that consist of all-caps letters:

	sub foo {
		print <<'EOF'
	text goes here
	...
	EOF
		... rest of foo ...
	}

That's not so great, because it means that "git diff" and "git grep
--show-function" would write "=EOF" or "@@ EOF" as context instead of
a more useful reminder like "@@ sub foo {".

To avoid this, tighten the pattern to only match the special block
names that perl accepts (namely BEGIN, END, INIT, CHECK, UNITCHECK,
AUTOLOAD, and DESTROY).  The list is taken from perl's toke.c.

Suggested-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>