index: do not warn about unrecognized extensions

Documentation/technical/index-format explains:

     4-byte extension signature. If the first byte is 'A'..'Z' the
     extension is optional and can be ignored.

This allows gracefully introducing a new index extension without
having to rely on all readers having support for it.  Mandatory
extensions start with a lowercase letter and optional ones start with
a capital.  Thus the versions of Git acting on a shared local
repository do not have to upgrade in lockstep.

We almost obey that convention, but there is a problem: when
encountering an unrecognized optional extension, we write

	ignoring FNCY extension

to stderr, which alarms users.  This means that in practice we have
had to introduce index extensions in two steps: first add read
support, and then a while later, start writing by default.  This
delays when users can benefit from improvements to the index format.

We cannot change the past, but for index extensions of the future,
there is a straightforward improvement: silence that message except
when tracing.  This way, the message is still available when
debugging, but in everyday use it does not show up so (once most Git
users have this patch) we can turn on new optional extensions right
away without alarming people.

Signed-off-by: Jonathan Nieder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jrn

read-cache.c

@@ -1726,7 +1726,7 @@ static int read_index_extension(struct index_state *istate,
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
 				     ext);
-		fprintf(stderr, "ignoring %.4s extension\n", ext);
+		trace_printf("ignoring %.4s extension\n", ext);
 		break;
 	}
 	return 0;