Allow underscores to appear in Harbormaster variable names

Summary:
See PHI859. Ref T13195. The regexp for replacing variables currently does not include underscores.

Include underscores.

I also made a note in T13088: we should (almost certainly?) throw immediately if you try to pass a bogus variable name as a custom parameter, but this is a slightly larger change.

Test Plan:
  - Made an "http request" build plan with `?x=${initiator.phid}&y={$some_variable}`.
  - Added `some_variable` as a parameter to the parameter collection.
  - Before patch: `initiator.phid` was replaced, but `some_variable` was not.
  - After patch: both variables are replaced.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19645

src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php

@@ -193,7 +193,7 @@ abstract class HarbormasterBuildStepImplementation extends Phobject {
    * @return string String with variables replaced safely into it.
    */
   protected function mergeVariables($function, $pattern, array $variables) {
-    $regexp = '@\\$\\{(?P<name>[a-z\\./-]+)\\}@';
+    $regexp = '@\\$\\{(?P<name>[a-z\\./_-]+)\\}@';
 
     $matches = null;
     preg_match_all($regexp, $pattern, $matches);