Revert "fmt-merge-msg: stop treating `master` specially"

Revert "fmt-merge-msg: stop treating `master` specially"

This reverts commit 489947cee5095b168cbac111ff7bd1eadbbd90dd, which
stopped treating merges into the 'master' branch as special when
preparing the default merge message.  As the goal was not to have
any single branch designated as special, it solved it by leaving the
"into <branchname>" at the end of the title of the default merge
message for any and all branches.  An obvious and easy alternative
to treat everybody equally could have been to remove it for every
branch, but that involves loss of information.

We'll introduce a new mechanism to let end-users specify merges into
which branches would omit the "into <branchname>" from the title of
the default merge message, and make the mechanism, when unconfigured,
treat the traditional 'master' special again, so all the changes to
the tests we made earlier will become unnecessary, as these tests
will be run without configuring the said new mechanism.

Signed-off-by: Junio C Hamano <gitster@pobox.com>