VcsCommand: Rename execute() to start()

In order to conform to QtcProcess API.

Change-Id: I71fcc03846f32cd3c2df39824ce3f6730c20ebbb
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/vcscommand.h

@@ -104,7 +104,7 @@ public:
     void addJob(const Utils::CommandLine &command, int timeoutS,
                 const Utils::FilePath &workingDirectory = {},
                 const Utils::ExitCodeInterpreter &interpreter = {});
-    void execute(); // Execute tasks asynchronously!
+    void start();
 
     void addFlags(unsigned f);