strbuf: add a case insensitive starts_with()

Check in a case insensitive manner if one string is a prefix of another
string.

This function is used in a subsequent commit.
Signed-off-by: NLars Schneider <larsxschneider@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -455,6 +455,7 @@ extern void (*get_warn_routine(void))(const char *warn, va_list params);
 extern void set_die_is_recursing_routine(int (*routine)(void));
 
 extern int starts_with(const char *str, const char *prefix);
+extern int istarts_with(const char *str, const char *prefix);
 
 /*
  * If the string "str" begins with the string found in "prefix", return 1.

strbuf.c

@@ -11,6 +11,15 @@ int starts_with(const char *str, const char *prefix)
 			return 0;
 }
 
+int istarts_with(const char *str, const char *prefix)
+{
+	for (; ; str++, prefix++)
+		if (!*prefix)
+			return 1;
+		else if (tolower(*str) != tolower(*prefix))
+			return 0;
+}
+
 int skip_to_optional_arg_default(const char *str, const char *prefix,
 				 const char **arg, const char *def)
 {