ALLOC_GROW: avoid -Wsign-compare warnings

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -825,7 +825,7 @@ static void emit_rewrite_diff(const char *name_a,
 
 struct diff_words_buffer {
 	mmfile_t text;
-	long alloc;
+	unsigned long alloc;
 	struct diff_words_orig {
 		const char *begin, *end;
 	} *orig;