FS_POSIX.runInShell(): on MacOS use a login shell

On Mac, $PATH for UI programs is not the same as in a shell (terminal).
It is typically much shorter. This may lead to surprises when hooks that
work fine via the command-line git do not work when run via JGit in a UI
application, such as EGit in Eclipse.

Therefore use a login shell to run hooks and other commands spawned by
git if we're on MacOS. This will give the hooks the same environment as
in a terminal.

Bug: egit-16
Change-Id: Id2e5485c6d3080d3ef8baa61ad7f6f198f77c590
Signed-off-by: Thomas Wolf <twolf@apache.org>