This tries to be more lenient to the users and stricter to the
attackers by quoting the input properly for shell safety,
instead of forbidding certain characters from the input.

Things to note:

 - We do not quote "prog" parameter (which comes from --exec).
   The user should know what he is doing.  --exec='echo foo'
   will supply the first two parameters to the resulting
   command, while --exec="'echo foo'" will give the first
   parameter, a single string with a space inside.

 - We do not care too much about leaking the sq_quote() output
   just before running exec().
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>