Update diff parsing for isort 5.5+ (#49)

PyCQA/isort#1429 fixed --check's output so
that the error message and diff output are separated into STDERR and
STDOUT rather than combined in STDOUT. This means we need to update our
diff parsing function accordingly.

Also fix the version parsing routine to read from STDOUT. This was
busted for at least a few versions.

src/PythonIsortLinter.php

@@ -58,7 +58,7 @@ public function getVersion() {
       $this->getExecutableCommand());
 
     $matches = array();
-    if (preg_match('/^(?P<version>\d+\.\d+\.\d+)$/', $stderr, $matches)) {
+    if (preg_match('/^(?P<version>\d+\.\d+\.\d+)$/', $stdout, $matches)) {
       $this->version = $matches['version'];
       return $this->version;
     } else {
@@ -89,13 +89,17 @@ protected function parseLinterOutput($path, $err, $stdout, $stderr) {
         return array();
     }
 
-    // Expected output includes a single header optionally followed by a
-    // multiline diff. We're only interested in the latter case (which implies
-    // a lint violation), but we could also parse the header for more detailed
-    // information if we need that later.
-    list($header, $diff) = explode("\n", $stdout, 2);
-    if (empty($diff)) {
+    if (version_compare($this->version, '5.5.0', '>=')) {
+      $diff = $stdout;
+    } else {
+      // Expected output includes a single header optionally followed by a
+      // multiline diff. We're only interested in the latter case (which implies
+      // a lint violation), but we could also parse the header for more detailed
+      // information if we need that later.
+      list($header, $diff) = explode("\n", $stdout, 2);
+      if (empty($diff)) {
         return array();
+      }
     }
 
     $messages = array();