reflog + refs-backend: move "verbose" out of the backend

Move the handling of the "verbose" flag entirely out of
"refs/files-backend.c" and into "builtin/reflog.c". This allows the
backend to stop knowing about the EXPIRE_REFLOGS_VERBOSE flag.

The expire_reflog_ent() function shouldn't need to deal with the
implementation detail of whether or not we're emitting verbose output,
by doing this the --verbose output becomes backend-agnostic, so
reftable will get the same output.

I think the output is rather bad currently, and should e.g. be
implemented with some better future mode of progress.[ch], but that's
a topic for another improvement.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -820,8 +820,7 @@ enum ref_type ref_type(const char *refname);
 enum expire_reflog_flags {
 	EXPIRE_REFLOGS_DRY_RUN = 1 << 0,
 	EXPIRE_REFLOGS_UPDATE_REF = 1 << 1,
-	EXPIRE_REFLOGS_VERBOSE = 1 << 2,
-	EXPIRE_REFLOGS_REWRITE = 1 << 3
+	EXPIRE_REFLOGS_REWRITE = 1 << 2,
 };
 
 /*