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>

Documentation/gitattributes.txt

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index c61a58d..db16b0c 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -310,6 +310,8 @@
 attribute mechanism, via `.gitattributes`).  The following built in
 patterns are available:
 
+- `bibtex` suitable for files with BibTeX coded references.
+
 - `java` suitable for source code in the Java lanugage.
 
 - `pascal` suitable for source code in the Pascal/Delphi language.

diff.c

diff --git a/diff.c b/diff.c
index 6954f99..bf5d5f1 100644
--- a/diff.c
+++ b/diff.c
@@ -1387,6 +1387,7 @@
 			"\\|"
 			"^\\(.*=[ \t]*\\(class\\|record\\).*\\)$"
 			},
+	{ "bibtex", "\\(@[a-zA-Z]\\{1,\\}[ \t]*{\\{0,1\\}[ \t]*[^ \t\"@',\\#}{~%]*\\).*$" },
 	{ "tex", "^\\(\\\\\\(\\(sub\\)*section\\|chapter\\|part\\)\\*\\{0,1\\}{.*\\)$" },
 	{ "ruby", "^\\s*\\(\\(class\\|module\\|def\\)\\s.*\\)$" },
 };