Add hook for runtime config settings.

Summary:
This adds a hook to allow external parties to provide config settings at runtime.
The hook is technically for when a RepositoryAPI is created, but that moment can
be used to set new config settings using the new setRuntimeConfig() api.

For example you could have a external hook that looks for keys like 'git:foo.bar'
or 'hg:foo.bar' and writes the value of 'foo.bar' based on whether the repo is a
git or a hg repo.

Test Plan:
Created a hook that looks for hg/git prefix versions of config keys.
Set hg:arc.feature.start.default to be "master" and set arc.feature.start.default
to be "trunk".
Ran arc feature in the hg repo.  It made a bookmark on master.
Ran arc feature in the git repo.  It made a branch on trunk.

Did it again, but with git:arc.feature... set instead.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, wez, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5357

src/workingcopyidentity/ArcanistWorkingCopyIdentity.php

@@ -12,6 +12,7 @@ final class ArcanistWorkingCopyIdentity {
 
   protected $localConfig;
   protected $projectConfig;
+  protected $runtimeConfig;
   protected $projectRoot;
 
   public static function newDummyWorkingCopy() {
@@ -87,6 +88,7 @@ protected function __construct($root, array $config) {
     $this->projectRoot    = $root;
     $this->projectConfig  = $config;
     $this->localConfig    = array();
+    $this->runtimeConfig = array();
 
     $vc_dirs = array(
       '.git',
@@ -214,8 +216,13 @@ public function getLocalConfig($key, $default=null) {
   public function getConfigFromAnySource($key, $default = null) {
     $settings = new ArcanistSettings();
 
-    // try local config first
-    $pval = $this->getLocalConfig($key);
+    // try runtime config first
+    $pval = idx($this->runtimeConfig, $key);
+
+    // try local config
+    if ($pval === null) {
+      $pval = $this->getLocalConfig($key);
+    }
 
     // then per-project config
     if ($pval === null) {
@@ -244,4 +251,19 @@ public function getConfigFromAnySource($key, $default = null) {
 
   }
 
+  /**
+   * Sets a runtime config value that takes precedence over any static
+   * config values.
+   *
+   * @param key   Key to set.
+   * @param value The value of the key.
+   *
+   * @task config
+   */
+  public function setRuntimeConfig($key, $value) {
+    $this->runtimeConfig[$key] = $value;
+
+    return $this;
+  }
+
 }