add shebang line to git-mergetool--lib.sh

add shebang line to git-mergetool--lib.sh

Even though this script is expected to be sourced instead of
executed on its own, the #!/bin/sh line provides simple
documentation about what format the file is in.

In particular, the lack of such a line 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 this file actually sourced the valgrind
interception script instead.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>