In "arc land" in Mercurial, show a tidier "ls-remote" command

Summary:
Ref T9948. Ref T13546. We must passthru "hg ls-remote" because it might prompt the user for credentials.

Since "ls-remote" is implemented as an exension and we can't rely on using stdout beacuse of passthru, the actual command we execute is:

```
$ hg --config extensions.arc-hg=<huge long path to extension> arc-ls-remote --output <huge long path to temporary file> -- remote
```

This is meaningless and distracting; show the intent of the command we're executing instead. Users can see the raw command in "--trace" if they're actually debugging behavior.

Test Plan: Ran "arc land" in a Mercurial repository, got a tidier command output.

Maniphest Tasks: T13546, T9948

Differential Revision: https://secure.phabricator.com/D21344

src/land/engine/ArcanistLandEngine.php

@@ -1547,7 +1547,7 @@ abstract class ArcanistLandEngine
     return $sets;
   }
 
-  final protected function newPassthru($pattern /* , ... */) {
+  final protected function newPassthruCommand($pattern /* , ... */) {
     $workflow = $this->getWorkflow();
     $argv = func_get_args();
 
@@ -1560,6 +1560,16 @@ abstract class ArcanistLandEngine
     $command = $workflow->newCommand($passthru)
       ->setResolveOnError(true);
 
+    return $command;
+  }
+
+  final protected function newPassthru($pattern /* , ... */) {
+    $argv = func_get_args();
+
+    $command = call_user_func_array(
+      array($this, 'newPassthruCommand'),
+      $argv);
+
     return $command->execute();
   }
 

src/land/engine/ArcanistMercurialLandEngine.php

@@ -455,18 +455,20 @@ final class ArcanistMercurialLandEngine
     // NOTE: We're using passthru on this because it's a remote command and
     // may prompt the user for credentials.
 
-    // TODO: This is fairly silly/confusing to show to users in the common
-    // case where it does not require credentials, particularly because the
-    // actual command line is full of nonsense.
-
     $tmpfile = new TempFile();
     Filesystem::remove($tmpfile);
 
-    $err = $this->newPassthru(
+    $command = $this->newPassthruCommand(
       '%Ls arc-ls-remote --output %s -- %s',
       $api->getMercurialExtensionArguments(),
       phutil_string_cast($tmpfile),
       $target->getRemote());
+
+    $command->setDisplayCommand(
+      'hg ls-remote -- %s',
+      $target->getRemote());
+
+    $err = $command->execute();
     if ($err) {
       throw new Exception(
         pht(

src/toolset/command/ArcanistCommand.php

@@ -6,6 +6,7 @@ final class ArcanistCommand
   private $logEngine;
   private $executableFuture;
   private $resolveOnError = false;
+  private $displayCommand;
 
   public function setExecutableFuture(PhutilExecutableFuture $future) {
     $this->executableFuture = $future;
@@ -34,10 +35,27 @@ final class ArcanistCommand
     return $this->resolveOnError;
   }
 
+  public function setDisplayCommand($pattern /* , ... */) {
+    $argv = func_get_args();
+    $command = call_user_func_array('csprintf', $argv);
+    $this->displayCommand = $command;
+    return $this;
+  }
+
+  public function getDisplayCommand() {
+    return $this->displayCommand;
+  }
+
   public function execute() {
     $log = $this->getLogEngine();
     $future = $this->getExecutableFuture();
+
+    $display_command = $this->getDisplayCommand();
+    if ($display_command !== null) {
+      $command = $display_command;
+    } else {
       $command = $future->getCommand();
+    }
 
     $log->writeNewline();