read-tree: add --quiet

read-tree is basically the front end of unpack-trees code and shoud
expose all of its functionality (unless it's designed for internal
use). This "opts.quiet" (formerly "opts.gently") was added for
builtin/checkout.c but there is no reason why other read-tree users
won't find this useful.

The test that is updated to run 'read-tree --quiet' was added because
unpack-trees was accidentally not being quiet [1] in 6a143aa2b2
(checkout -m: attempt merge when deletion of path was staged -
2014-08-12). Because checkout is the only "opts.quiet" user, there was
no other way to test quiet behavior. But we can now test it directly.

6a143aa2b2 was manually reverted to verify that read-tree --quiet
works correctly (i.e. test_must_be_empty fails).

[1] the commit message there say "errors out instead of performing a
    merge" but I'm pretty sure the "performing a merge" happens anyway
    even before that commit. That line should say "errors out
    _in addition to_ performing a merge"

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-read-tree.txt

@@ -128,6 +128,10 @@ OPTIONS
 	Instead of reading tree object(s) into the index, just empty
 	it.
 
+-q::
+--quiet::
+	Quiet, suppress feedback messages.
+
 <tree-ish#>::
 	The id of the tree object(s) to be read/merged.
 

builtin/read-tree.c

@@ -154,6 +154,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		{ OPTION_CALLBACK, 0, "recurse-submodules", NULL,
 			    "checkout", "control recursive updating of submodules",
 			    PARSE_OPT_OPTARG, option_parse_recurse_submodules_worktree_updater },
+		OPT__QUIET(&opts.quiet, N_("suppress feedback messages")),
 		OPT_END()
 	};
 

t/t7201-co.sh

@@ -223,6 +223,9 @@ test_expect_success 'switch to another branch while carrying a deletion' '
 	test_must_fail git checkout simple 2>errs &&
 	test_i18ngrep overwritten errs &&
 
+	test_must_fail git read-tree --quiet -m -u HEAD simple 2>errs &&
+	test_must_be_empty errs &&
+
 	git checkout --merge simple 2>errs &&
 	test_i18ngrep ! overwritten errs &&
 	git ls-files -u &&