Git: Fix ability to open file on double click

since show can be called with file name as source, we get its
repository as source in diffhandler

Task-number: QTCREATORBUG-10780
Change-Id: Ib0076c583eb5d43e66deb68adfdff1d25fc5a798
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/gitclient.cpp

@@ -1443,7 +1443,7 @@ void GitClient::show(const QString &source, const QString &id,
 
         GitDiffHandler *handler = new GitDiffHandler(diffEditor,
                                                      gitBinaryPath(),
-                                                     workingDirectory,
+                                                     findRepositoryForDirectory(workingDirectory),
                                                      processEnvironment(),
                                                      settings()->intValue(GitSettings::timeoutKey));
         handler->show(id);