VcsBase: Introduce vcsExecWithHandler()

Before, vcsExec() returned already started VcsCommand.
Later, callers of vcsExec() were establishing connections
to the retured VcsCommand::done() signal. However, when
process fails to start (e.g. because of non-existing
executable), the done() signal may be emitted synchonously
from inside VcsCommand::start(). In this scenario
callers of VcsCommand could miss the emission of done()
signal and connect to already finished command.

Instead, provide a vcsExecWithHandler() function which
takes a handler to be called when command finished.
In addition it takes the context object, too.
Don't return VcsCommand from vcsExec() anymore.

Change-Id: I2fb5fbe5d27632ea039c650d37e5d7d1b60cebc0
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/gitsubmiteditor.cpp

@@ -203,6 +203,7 @@ void GitSubmitEditor::updateFileModel()
     if (w->updateInProgress() || m_workingDirectory.isEmpty())
         return;
     w->setUpdateInProgress(true);
+    // TODO: Check if fetch works OK from separate thread, refactor otherwise
     m_fetchWatcher.setFuture(Utils::runAsync(&CommitDataFetchResult::fetch,
                                              m_commitType, m_workingDirectory));
     Core::ProgressManager::addTask(m_fetchWatcher.future(), Tr::tr("Refreshing Commit Data"),