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.cpp

@@ -333,4 +333,10 @@ CommandResult::CommandResult(const QtcProcess &process)
     , m_rawStdOut(process.rawStdOut())
 {}
 
+CommandResult::CommandResult(const VcsCommand &command)
+    : m_result(command.result())
+    , m_cleanedStdOut(command.cleanedStdOut())
+    , m_cleanedStdErr(command.cleanedStdErr())
+{}
+
 } // namespace VcsBase