help: format ``commands`` heading correctly
The number of dashes under it needs to match exactly for it to be
rendered as a heading. Without this change, the dashes end up on the
same line as "commands", and "hg help config.commands" does not work.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown