Make external diff command work again

.. by fixing SHA1 inspection in method copyFileToDiffIfNeeded
where any SHA1 matched regex pattern "0*".

That led the external diff feature to always compare the working
directory state of a file against itself rather than comparing
the version from a given commit against the version from the commit'
parent.

Regression from commit 1cb9acf

Author: hartwork

src/mainimpl.cpp

@@ -291,10 +291,8 @@ void MainImpl::ActExternalDiff_activated() {
 	}
 }
 
-const QRegularExpression MainImpl::emptySha("0*");
-
 QString MainImpl::copyFileToDiffIfNeeded(QStringList* filenames, QString sha) {
-	if (emptySha.match(sha).hasMatch())
+	if (sha == QGit::ZERO_SHA)
 	{
 		return QString(curDir + "/" + rv->st.fileName());
 	}

src/mainimpl.h

@@ -187,7 +187,6 @@ protected slots:
 	QString textToFind;
 	QRegularExpression shortLogRE;
 	QRegularExpression longLogRE;
-	static const QRegularExpression emptySha;
 	QMap<QString, QVariant> revision_variables; // variables used in generic input dialogs
 	bool setRepositoryBusy;