range-diff: libify the read_patches() function again

In library functions, we do want to avoid the (simple, but rather final)
`die()` calls, instead returning with a value indicating an error.

Let's do exactly that in the code introduced in b66885a
(range-diff: add section header instead of diff header, 2019-07-11) that
wants to error out if a diff header could not be parsed.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

range-diff.c

@@ -135,9 +135,16 @@ static int read_patches(const char *range, struct string_list *list,
 			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'"),
-				    orig_len, line);
+			if (len < 0) {
+				error(_("could not parse git header '%.*s'"),
+				      orig_len, line);
+				free(util);
+				free(current_filename);
+				string_list_clear(list, 1);
+				strbuf_release(&buf);
+				strbuf_release(&contents);
+				return -1;
+			}
 			strbuf_addstr(&buf, " ## ");
 			if (patch.is_new > 0)
 				strbuf_addf(&buf, "%s (new)", patch.new_name);