Support mustache partials in server-side templates

This is sufficient to make https://gerrit.wikimedia.org/r/#/c/223165/
work.  It hardcodes .mustache as the extension, but so does
existing getTemplateFilename().

Bug: T97188
Change-Id: Id588ae9b43b13fcf35ebd285c826dd502ac424ec

includes/TemplateParser.php

@@ -173,7 +173,9 @@ class TemplateParser {
 			array(
 				// Do not add more flags here without discussion.
 				// If you do add more flags, be sure to update unit tests as well.
-				'flags' => LightnCandy::FLAG_ERROR_EXCEPTION
+				'flags' => LightnCandy::FLAG_ERROR_EXCEPTION,
+				'basedir' => $this->templateDir,
+				'fileext' => '.mustache',
 			)
 		);
 	}

tests/phpunit/data/templates/bad_partial.mustache

@@ -0,0 +1 @@
+Partial {{>nonexistenttemplate}} in here

tests/phpunit/data/templates/has_partial.mustache

@@ -0,0 +1 @@
+Partial {{>foobar_args}} in here

tests/phpunit/includes/TemplateParserTest.php

@@ -57,7 +57,20 @@ class TemplateParserTest extends MediaWikiTestCase {
 				array(),
 				false,
 				'RuntimeException',
-			)
+			),
+			array(
+				'has_partial',
+				array(
+					'planet' => 'world',
+				),
+				"Partial hello world!\n in here\n",
+			),
+			array(
+				'bad_partial',
+				array(),
+				false,
+				'Exception',
+			),
 		);
 	}
 }