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/vcsbase/diffhighlighter.cpp

@@ -116,7 +116,7 @@ DiffFormats DiffHighlighterPrivate::analyzeLine(const QString &text) const
 {
     // Do not match on git "--- a/" as a deleted line, check
     // file first
-    if (m_filePattern.exactMatch(text))
+    if (m_filePattern.indexIn(text) == 0)
         return DiffFileFormat;
     if (text.startsWith(m_diffInIndicator))
         return DiffInFormat;