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

lib/ansible/modules/commands/command.py

@@ -183,7 +183,8 @@ 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']
@@ -225,6 +226,15 @@ def main():
                 changed=False,
                 rc=0
             )
+        # if in check mode and the filename doesnt exist, report that a change
+        # would be made
+        elif module.check_mode:
+            module.exit_json(
+                cmd=args,
+                stdout="changed, since %s does not exist" % creates,
+                changed=True,
+                rc=0
+            )
 
     if removes:
         # do not run the command if the line contains removes=filename
@@ -237,10 +247,23 @@ def main():
                 changed=False,
                 rc=0
             )
+        # if in check mode and the filename exists, report that a change would
+        # be made
+        elif module.check_mode:
+            module.exit_json(
+                cmd=args,
+                stdout="changed, since %s exists" % removes,
+                changed=True,
+                rc=0
+            )
 
     if warn:
         check_command(module, args)
 
+    # skip if in check mode so no changes are made
+    if module.check_mode:
+        module.exit_json(msg="skipped, running in check mode", skipped=True)
+
     startd = datetime.datetime.now()
 
     rc, out, err = module.run_command(args, executable=executable, use_unsafe_shell=shell, encoding=None, data=stdin)