range-diff: avoid negative string precision

If the supplied integer for "precision" is negative in
`"%.*s", len, line` then it is ignored. So the current code is
equivalent to just `"%s", line` because it is executed only if
`len` is negative.

Fix this by saving the value of `len` before overwriting it with the
return value of `parse_git_diff_header()`.

Signed-off-by: Vasil Dimov <vd@FreeBSD.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -123,16 +123,19 @@ static int read_patches(const char *range, struct string_list *list,
 			struct patch patch = { 0 };
 			struct strbuf root = STRBUF_INIT;
 			int linenr = 0;
+			int orig_len;
 
 			in_header = 0;
 			strbuf_addch(&buf, '\n');
 			if (!util->diff_offset)
 				util->diff_offset = buf.len;
 			line[len - 1] = '\n';
+			orig_len = len;
 			len = parse_git_diff_header(&root, &linenr, 0, line,
 						    len, size, &patch);
 			if (len < 0)
-				die(_("could not parse git header '%.*s'"), (int)len, line);
+				die(_("could not parse git header '%.*s'"),
+				    orig_len, line);
 			strbuf_addstr(&buf, " ## ");
 			if (patch.is_new > 0)
 				strbuf_addf(&buf, "%s (new)", patch.new_name);