Makefile: do not install sourced rebase scripts

Makefile: do not install sourced rebase scripts

When git-rebase.sh recently started sourcing
git-rebase--interactive.sh instead of executing it, executable bit of
the latter file should have been turned off and it should have been
moved from SCRIPT_SH to SCRIPT_LIB in the Makefile. Its two new
siblings, git-rebase--am.sh and git-rebase--merge.sh (whose executable
bits are already off) should also be moved to SCRIPT_LIB in the
Makefile.

Reported-by: Johannes Sixt <j6t@kdbg.org>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>