submodule update: silence underlying merge/rebase with "--quiet"

Commands such as

    $ git pull --rebase --recurse-submodules --quiet

produce non-quiet output from the merge or rebase.  Pass the --quiet
option down when invoking "rebase" and "merge".

Also fix the parsing of git submodule update -v.

When e84c3cf3 (git-submodule.sh: accept verbose flag in cmd_update
to be non-quiet, 2018-08-14) taught "git submodule update" to take
"--quiet", it apparently did not know how ${GIT_QUIET:+--quiet}
works, and reviewers seem to have missed that setting the variable
to "0", rather than unsetting it, still results in "--quiet" being
passed to underlying commands.

Signed-off-by: Theodore Dubois <tbodt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -465,7 +465,7 @@ cmd_update()
 			GIT_QUIET=1
 			;;
 		-v)
-			GIT_QUIET=0
+			unset GIT_QUIET
 			;;
 		--progress)
 			progress=1
@@ -639,13 +639,13 @@ cmd_update()
 				say_msg="$(eval_gettext "Submodule path '\$displaypath': checked out '\$sha1'")"
 				;;
 			rebase)
-				command="git rebase"
+				command="git rebase ${GIT_QUIET:+--quiet}"
 				die_msg="$(eval_gettext "Unable to rebase '\$sha1' in submodule path '\$displaypath'")"
 				say_msg="$(eval_gettext "Submodule path '\$displaypath': rebased into '\$sha1'")"
 				must_die_on_failure=yes
 				;;
 			merge)
-				command="git merge"
+				command="git merge ${GIT_QUIET:+--quiet}"
 				die_msg="$(eval_gettext "Unable to merge '\$sha1' in submodule path '\$displaypath'")"
 				say_msg="$(eval_gettext "Submodule path '\$displaypath': merged in '\$sha1'")"
 				must_die_on_failure=yes

t/t7406-submodule-update.sh

@@ -1022,4 +1022,16 @@ test_expect_success 'git clone passes the parallel jobs config on to submodules'
 	rm -rf super4
 '
 
+test_expect_success 'submodule update --quiet passes quietness to merge/rebase' '
+	(cd super &&
+	 test_commit -C rebasing message &&
+	 git submodule update --rebase --quiet >out 2>err &&
+	 test_must_be_empty out &&
+	 test_must_be_empty err &&
+	 git submodule update --rebase -v >out 2>err &&
+	 test_file_not_empty out &&
+	 test_must_be_empty err
+	)
+'
+
 test_done