fix(session): chain the working directory with && instead of ;

[loicsapone]

Commands were sent as:

sprintf('cd %s; %s', $this->getReleasePath(), $command)

; runs the second command whatever happened to the first. When the release directory
is missing — a cleanup that ran concurrently, a release moved to releases/failed
after an earlier failure — the cd fails but the command still runs, from the home
directory of the SSH user. A hook such as rm -rf var/cache then hits the wrong tree.

Both exec() and execAsync() now use &&.

Tests

Existing assertions updated, plus a case covering execAsync() with a working
directory, which had the same bug and no coverage.

Note

The paths themselves are still interpolated unquoted, here and in
mkdir/mv/rm/symlink/touch. Escaping them is a consistent change of its own
and is deliberately left out of this PR.

[julienj]

Thank @loicsapone