Utils: Add CommandLine convenience constructors

... taking a QString for the executable.

This weakens the very explicit QString -> FileName conversion via the
named constructors for the special case of constructing a CommandLine.

I think that's worthwhile here, as it reduces the noise on the caller
site under circumstance where the nature of the thing is obvious.

Change-Id: I27b4a73639728893d053b2e7ba65cb745f0ffe83
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clearcase/clearcaseplugin.cpp

@@ -1453,7 +1453,7 @@ ClearCasePlugin::runCleartool(const QString &workingDir,
 
     const SynchronousProcessResponse sp_resp =
             VcsBasePlugin::runVcs(workingDir,
-                                  {FilePath::fromUserInput(executable), arguments},
+                                  {executable, arguments},
                                   timeOutS,
                                   flags, outputCodec);
 
@@ -2141,7 +2141,7 @@ void ClearCasePlugin::diffGraphical(const QString &file1, const QString &file2)
 QString ClearCasePlugin::runExtDiff(const QString &workingDir, const QStringList &arguments,
                                     int timeOutS, QTextCodec *outputCodec)
 {
-    CommandLine diff(FilePath::fromString("diff"));
+    CommandLine diff("diff");
     diff.addArgs(m_settings.diffArgs.split(' ', QString::SkipEmptyParts));
     diff.addArgs(arguments);