Enable check_mode in command module (#40428)

* Enable check_mode in command module

This only works if supplying creates or removes since it needs
something to base the heuristic off. If none are supplied it will just
skip as usual.
Fixes #15828

* Add documentation for new check_mode behavior

lib/ansible/modules/commands/command.py

@@ -64,6 +64,8 @@ notes:
        use the C(command) module when possible.
     -  " C(creates), C(removes), and C(chdir) can be specified after the command.
        For instance, if you only want to run a command if a certain file does not exist, use this."
+    -  Check mode is supported when passing C(creates) or C(removes). If running in check mode and either of these are specified, the module will
+       check for the existence of the file and report the correct changed status. If these are not supplied, the task will be skipped.
     -  The C(executable) parameter is removed since version 2.4. If you have a need for this parameter, use the M(shell) module instead.
     -  For Windows targets, use the M(win_command) module instead.
 author:
@@ -185,7 +187,8 @@ def main():
             # The default for this really comes from the action plugin
             warn=dict(type='bool', default=True),
             stdin=dict(required=False),
-        )
+        ),
+        supports_check_mode=True,
     )
     shell = module.params['_uses_shell']
     chdir = module.params['chdir']
@@ -245,7 +248,13 @@ def main():
 
     startd = datetime.datetime.now()
 
-    rc, out, err = module.run_command(args, executable=executable, use_unsafe_shell=shell, encoding=None, data=stdin)
+    if not module.check_mode:
+        rc, out, err = module.run_command(args, executable=executable, use_unsafe_shell=shell, encoding=None, data=stdin)
+    elif creates or removes:
+        rc = 0
+        out = err = b'Command would have run if not in check mode'
+    else:
+        module.exit_json(msg="skipped, running in check mode", skipped=True)
 
     endd = datetime.datetime.now()
     delta = endd - startd