improve git error logging and fix deprecations

Author: IonBazan

src/PackageDiff.php

@@ -130,11 +130,12 @@ private function getFileContents($path)
 
         $output = array();
         @exec(sprintf('git show %s 2>&1', escapeshellarg($path)), $output, $exit);
+        $outputString = implode("\n", $output);
 
         if (0 !== $exit) {
-            throw new \RuntimeException(sprintf('Could not open file %s or find it in git as %s', $originalPath, $path));
+            throw new \RuntimeException(sprintf('Could not open file %s or find it in git as %s: %s', $originalPath, $path, $outputString));
         }
 
-        return implode("\n", $output);
+        return $outputString;
     }
 }

src/Url/GitGenerator.php

@@ -11,7 +11,7 @@ abstract class GitGenerator implements UrlGenerator
      */
     public function supportsPackage(PackageInterface $package)
     {
-        return false !== strpos($package->getSourceUrl(), $this->getDomain());
+        return false !== strpos((string) $package->getSourceUrl(), $this->getDomain());
     }
 
     /**