doc txt & -h consistency: make "stash" consistent

doc txt & -h consistency: make "stash" consistent

Amend both the -h output and *.txt to match one another. In this case
the *.txt didn't list the "save" subcommand, and the "-h" was
similarly missing some commands.

Let's also convert the *.c code to use a macro definition, similar to
that used in preceding commits. This avoids duplication.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>