submodule: if $command was not matched, don't parse other args

"git submodule" command DWIMs the command line and assumes a
unspecified action word for 'status' action.  This is a UI mistake
that leads to a confusing behaviour.  A mistyped command name is
instead treated as a request for 'status' of the submodule with that
name, e.g.

    $ git submodule show
    error: pathspec 'show' did not match any file(s) known to git.
    Did you forget to 'git add'?

Stop DWIMming an unknown or mistyped subcommand name as pathspec
given to unspelled "status" subcommand.  "git submodule" without any
argument is still interpreted as "git submodule status", but its
value is questionable.

Adjust t7400 to match, and stop advertising the default subcommand
being 'status' which does not help much in practice, other than
promoting laziness and confusion.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -112,7 +112,6 @@ status::
 	initialized, `+` if the currently checked out submodule commit
 	does not match the SHA-1 found in the index of the containing
 	repository and `U` if the submodule has merge conflicts.
-	This command is the default command for 'git submodule'.
 +
 If `--recursive` is specified, this command will recurse into nested
 submodules, and show their status as well.

git-submodule.sh

@@ -1056,7 +1056,15 @@ do
 done
 
 # No command word defaults to "status"
-test -n "$command" || command=status
+if test -z "$command"
+then
+    if test $# = 0
+    then
+	command=status
+    else
+	usage
+    fi
+fi
 
 # "-b branch" is accepted only by "add"
 if test -n "$branch" && test "$command" != add

t/t7400-submodule-basic.sh

@@ -438,8 +438,8 @@ test_expect_success 'moving to a commit without submodule does not leave empty d
 	git checkout second
 '
 
-test_expect_success 'submodule <invalid-path> warns' '
-	test_failure_with_unknown_submodule
+test_expect_success 'submodule <invalid-subcommand> fails' '
+	test_must_fail git submodule no-such-subcommand
 '
 
 test_expect_success 'add submodules without specifying an explicit path' '