scm: use shell quote for scm command at adapter level (#7517, #4273).

"C:\Program Files\TortoiseHg\hg.exe" can be used in config/configuration.yml.

In Ruby 1.9 IO.popen, if cmd is an Array of String,
it will be used as the subprocess‘s argv bypassing a shell.

See http://www.ruby-doc.org/core/classes/IO.html

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@4821 e93f8b46-1217-0410-a6f0-8f06a7374b81