Add a --whatif option to CLI update command

This flag will cause the command to print a message indicating what
changes would be made, but not commit any changes to the file system.
It's useful to plan in advance before making changes.

Author: jimmylewis

src/libman/Commands/UpdateCommand.cs

@@ -48,6 +48,11 @@ public UpdateCommand(IHostEnvironment environment, bool throwOnUnexpectedArg = t
         /// <remarks>Needs the full library id.</remarks>
         public CommandOption ToVersion { get; private set; }
 
+        /// <summary>
+        /// Option to specify if this should print the operation that would be carried out, but not make changes.
+        /// </summary>
+        public CommandOption WhatIf { get; private set; }
+
         public override BaseCommand Configure(CommandLineApplication parent = null)
         {
             base.Configure(parent);
@@ -56,6 +61,7 @@ public override BaseCommand Configure(CommandLineApplication parent = null)
             Provider = Option("--provider|-p", Resources.Text.UpdateCommandProviderOptionDesc, CommandOptionType.SingleValue);
             PreRelease = Option("-pre", Resources.Text.UpdateCommandPreReleaseOptionDesc, CommandOptionType.NoValue);
             ToVersion = Option("--to", Resources.Text.UpdateCommandToVersionOptionDesc, CommandOptionType.SingleValue);
+            WhatIf = Option("--whatif", Resources.Text.WhatIfOptionDesc, CommandOptionType.NoValue);
 
             // Reserve this.
             Provider.ShowInHelpText = false;
@@ -106,6 +112,12 @@ protected override async Task<int> ExecuteInternalAsync()
                 return 0;
             }
 
+            if (WhatIf.HasValue())
+            {
+                Logger.Log(string.Format(Resources.Text.WhatIfOutputMessage, libraryToUpdate.Name, newVersion), LogLevel.Operation);
+                return 0;
+            }
+
             Manifest backup = manifest.Clone();
             string oldLibraryName = libraryToUpdate.Name;
             Manifest.UpdateLibraryVersion(libraryToUpdate, newVersion);

src/libman/Resources/Text.Designer.cs

@@ -431,4 +431,10 @@ Please specify a different destination.</value>
   <data name="UseDefault" xml:space="preserve">
     <value>Use the default settings for the libman.json file</value>
   </data>
+  <data name="WhatIfOptionDesc" xml:space="preserve">
+    <value>Print what operations would be carried out, without making any changes.</value>
+  </data>
+  <data name="WhatIfOutputMessage" xml:space="preserve">
+    <value>Library "{0}" would be updated to latest version "{1}"</value>
+  </data>
 </root>