Do not install shell libraries executable

Do not install shell libraries executable

Some scripts are expected to be sourced instead of executed on their own.
Avoid some confusion by not marking them executable.

The executable bit was confusing the valgrind support of our test scripts,
which assumed that any executable without a #!-line should be intercepted
and run through valgrind.  So during valgrind-enabled tests, any script
sourcing these files actually sourced the valgrind interception script
instead.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>