VcsCommand: Don't derive from ShellCommand

Transform VcsCommand into VcsCommandDecorator and
attach it to ShellCommand. There is no need to derive
from ShellCommand anymore.

Use ShellCommand's public API to setup vcs command via
VcsCommandDecorator.

Make ShellCommand final.

Change-Id: I39e3699c80a9e108bcaeef7fb02214e2cc0d0dee
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/libs/utils/shellcommand.cpp

@@ -162,6 +162,11 @@ const FilePath &ShellCommand::defaultWorkingDirectory() const
     return d->m_defaultWorkingDirectory;
 }
 
+Environment ShellCommand::environment() const
+{
+    return d->m_environment;
+}
+
 void ShellCommand::setEnvironment(const Environment &env)
 {
     d->m_environment = env;