The asciidoctor doc-tool stack does not always respect the 'tab = 8 spaces' rule
expectation, particularly for the Git-for-Windows generated html pages. This
follows on from the 'doc: fix merge-base ASCII art tab spacing' fix.

Use just spaces within the block of the ascii art.

All other *.txt ascii art containing three dashes has been checked.
Asciidoctor correctly formats the other art blocks that do contain tabs.

Signed-off-by: Philip Oakley <philipoakley@iee.org
Signed-off-by: NJunio C Hamano <gitster@pobox.com>