builtin/repack.c: extract showing progress to a variable

We only ask whether stderr is a tty before calling
'prune_packed_objects()', but the subsequent patch will add another use.

Extract this check into a variable so that both can use it without
having to call 'isatty()' twice.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/repack.c

@@ -446,6 +446,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	struct strbuf line = STRBUF_INIT;
 	int i, ext, ret;
 	FILE *out;
+	int show_progress = isatty(2);
 
 	/* variables to be filled by option parsing */
 	int pack_everything = 0;
@@ -719,7 +720,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 			}
 			strbuf_release(&buf);
 		}
-		if (!po_args.quiet && isatty(2))
+		if (!po_args.quiet && show_progress)
 			opts |= PRUNE_PACKED_VERBOSE;
 		prune_packed_objects(opts);