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/vcsbase/vcscommand.h

@@ -27,11 +27,14 @@ namespace VcsBase {
 
 namespace Internal { class VcsCommandPrivate; }
 
+class VcsCommand;
+
 class VCSBASE_EXPORT CommandResult
 {
 public:
     CommandResult() = default;
     CommandResult(const Utils::QtcProcess &process);
+    CommandResult(const VcsCommand &command);
     CommandResult(Utils::ProcessResult result, const QString &exitMessage)
         : m_result(result), m_exitMessage(exitMessage) {}