refactored "svn --version" calls into a single place, closes #7152

this saves a lot of process-spawning as we re-use the result of a process started once.

src/Composer/Downloader/SvnDownloader.php

@@ -57,11 +57,10 @@ class SvnDownloader extends VcsDownloader
             throw new \RuntimeException('The .svn directory is missing from '.$path.', see https://getcomposer.org/commit-deps for more information');
         }
 
+        $util = new SvnUtil($url, $this->io, $this->config);
         $flags = "";
-        if (0 === $this->process->execute('svn --version', $output)) {
-            if (preg_match('{(\d+(?:\.\d+)+)}', $output, $match) && version_compare($match[1], '1.7.0', '>=')) {
-                $flags .= ' --ignore-ancestry';
-            }
+        if (version_compare($util->binaryVersion(), '1.7.0', '>=')) {
+            $flags .= ' --ignore-ancestry';
         }
 
         $this->io->writeError(" Checking out " . $ref);

src/Composer/Repository/Vcs/SvnDriver.php

@@ -353,7 +353,7 @@ class SvnDriver extends VcsDriver
         try {
             return $this->util->execute($command, $url);
         } catch (\RuntimeException $e) {
-            if (0 !== $this->process->execute('svn --version', $ignoredOutput)) {
+            if (null === $this->util->binaryVersion()) {
                 throw new \RuntimeException('Failed to load '.$this->url.', svn was not found, check that it is installed and in your PATH env.' . "\n\n" . $this->process->getErrorOutput());
             }
 

src/Composer/Util/Svn.php

@@ -63,6 +63,11 @@ class Svn
      */
     protected $config;
 
+    /**
+     * @var string|null
+     */
+    static private $version;
+
     /**
      * @param string                   $url
      * @param \Composer\IO\IOInterface $io
@@ -359,4 +364,22 @@ class Svn
 
         return $this->hasAuth = true;
     }
+
+    /**
+     * Returns the version of the svn binary contained in PATH
+     *
+     * @return string|null
+     */
+    public function binaryVersion()
+    {
+        if (!self::$version) {
+            if (0 === $this->process->execute('svn --version', $output)) {
+                if (preg_match('{(\d+(?:\.\d+)+)}', $output, $match)) {
+                    self::$version = $match[1];
+                }
+            }
+        }
+
+        return self::$version;
+    }
 }