ws_fix_copy(): move the whitespace fixing function to ws.c

This is used by git-apply but we can use it elsewhere by slightly
generalizing it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -661,6 +661,7 @@ extern unsigned check_and_emit_line(const char *line, int len, unsigned ws_rule,
     FILE *stream, const char *set,
     const char *reset, const char *ws);
 extern char *whitespace_error_string(unsigned ws);
+extern int ws_fix_copy(char *, const char *, int, unsigned, int *);
 
 /* ls-files */
 int pathspec_match(const char **spec, char *matched, const char *filename, int skiplen);