Teach git diff about BibTeX head hunk patterns

All BibTeX entries starts with an @ followed by an entry type.  Since
there are many entry types and own can be defined, the pattern matches
legal entry type names instead of just the default types (which would
be a long list).  The pattern also matches strings and comments since
they will also be useful to position oneself in a bib-file.

Signed-off-by: Gustaf Hendeby <hendeby@isy.liu.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1387,6 +1387,7 @@ static struct builtin_funcname_pattern {
 			"\\|"
 			"^\\(.*=[ \t]*\\(class\\|record\\).*\\)$"
 			},
+	{ "bibtex", "\\(@[a-zA-Z]\\{1,\\}[ \t]*{\\{0,1\\}[ \t]*[^ \t\"@',\\#}{~%]*\\).*$" },
 	{ "tex", "^\\(\\\\\\(\\(sub\\)*section\\|chapter\\|part\\)\\*\\{0,1\\}{.*\\)$" },
 	{ "ruby", "^\\s*\\(\\(class\\|module\\|def\\)\\s.*\\)$" },
 };