Merge branch 'po/fix-doc-merge-base-illustration' into maint

Some AsciiDoc formatter mishandles a displayed illustration with
tabs in it.  Adjust a few of them in merge-base documentation to
work around them.

* po/fix-doc-merge-base-illustration:
doc: fix the 'revert a faulty merge' ASCII art tab spacing
doc: fix merge-base ASCII art tab spacing