strbuf: add xstrdup_toupper()

Create a copy of an existing string and make all characters upper case.
Similar xstrdup_tolower().

This function is used in a subsequent commit.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -607,6 +607,7 @@ __attribute__((format (printf,2,3)))
 extern int fprintf_ln(FILE *fp, const char *fmt, ...);
 
 char *xstrdup_tolower(const char *);
+char *xstrdup_toupper(const char *);
 
 /**
  * Create a newly allocated string using printf format. You can do this easily