git-merge-base.txt: render indentations correctly under Asciidoctor

git-merge-base.txt: render indentations correctly under Asciidoctor

There are several graphs in this document. For most of them, we use a
single leading tab to indent the whole graph, and then we use spaces
(possibly eight or more) to align things within the graph.

In the larger graph, we use a different strategy: We use 1-N tabs and
just a small number of spaces (<8). This is how we usually prefer to do
our indenting, but Asciidoctor ends up rendering this differently from
AsciiDoc. Same thing for the if-then-fi examples where the conditional
code is indented by two tabs, which renders differently under AsciiDoc
and Asciidoctor.

Similar to 379805051d ("Documentation: render revisions correctly under
Asciidoctor", 2018-05-06), use an explicit literal block to indicate
that we want to keep the leading whitespace in the tables. Change not
just the ones that render differently, but all of them for consistency.

Because this gives us some extra indentation, we can remove the one that
we have been carrying explicitly. That is, drop the first tab of
indentation on each line. With AsciiDoc, this results in identical
rendering before and after this commit, both for git-merge-base.1 and
git-merge-base.html.

A less intrusive change would be to replace tabs 2-N on each line with
eight spaces. But let's follow the example set by 379805051d, so that we
can use our preferred way of indenting.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>