rev-list-options.txt: start a list for `show-pulls`

rev-list-options.txt: start a list for `show-pulls`

The explanation of the `--show-pulls` option added in commit 8d049e182e
("revision: --show-pulls adds helpful merges", 2020-04-10) consists of
several paragraphs and we use "+" throughout to tie them together in one
long chain of list continuations. Only thing is, we're not in any kind
of list, so these pluses end up being rendered literally.

The preceding few paragraphs describe `--ancestry-path` and there we
*do* have a list, since we've started one with `--ancestry-path::`. In
fact, we have several such lists for all the various history-simplifying
options we're discussing earlier in this file.

Thus, we're missing a list both from a consistency point of view and
from a practical rendering standpoint.

Let's start a list for `--show-pulls` where we start actually discussing
the option, and keep the paragraphs preceding it out of that list. That
is, drop all those pluses before the new list we're adding here.

Helped-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>