VCS: Refactor fileNameFromDiffSpecification

Use diff file pattern to match file name. Avoid duplication.

Include unit tests for Git

Change-Id: Ib68a08368270a27976a3e16bdd1cb219a52b8889
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitplugin.h

@@ -144,6 +144,8 @@ private slots:
 #ifdef WITH_TESTS
     void testStatusParsing_data();
     void testStatusParsing();
+    void testDiffFileResolving_data();
+    void testDiffFileResolving();
 #endif
 protected:
     void updateActions(VcsBase::VcsBasePlugin::ActionState);