rebase: add a config option for --rebase-merges

rebase: add a config option for --rebase-merges

The purpose of the new option is to accommodate users who would like
--rebase-merges to be on by default and to facilitate turning on
--rebase-merges by default without configuration in a future version of
Git.

Name the new option rebase.rebaseMerges, even though it is a little
redundant, for consistency with the name of the command line option and
to be clear when scrolling through values in the [rebase] section of
.gitconfig.

Support setting rebase.rebaseMerges to the nonspecific value "true" for
users who don't need to or don't want to learn about the difference
between rebase-cousins and no-rebase-cousins.

Make --rebase-merges without an argument on the command line override
any value of rebase.rebaseMerges in the configuration, for consistency
with other command line flags with optional arguments that have an
associated config option.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>