The doc-tool stack does not always respect the 'tab = 8 spaces' rule,
particularly the git-scm doc pages https://git-scm.com/docs/git-merge-base
and the Git generated html pages.
Use just spaces within the block of the ascii art.
Noticed when reviewing Junio's suggested update to `git merge-base`
https://public-inbox.org/git/xmqqmvi2sj8f.fsf@gitster.mtv.corp.google.com/T/#u
Signed-off-by: Philip Oakley <philipoakley@iee.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>