[BUG] Regression in 'git mergetool --tool-help'

Hi all,

It seems that 83bbf9b92e (mergetool--lib: improve support for
vimdiff-style tool variants,
2020-07-29) introduced a regression in the output shown by 'git
mergetool --tool-help'.
Even if I have 'meld', 'kdiff3' and 'xxdiff' installed and in PATH,
they are not shown at all when
the above command is invoked. Only vimdiff, gvimdiff and their *2 and
*3 variants are listed.

The commit above was found by bisecting starting from 2.29.2 knowing
2.26.0 is OK. I can
reproduce the bug on Ubuntu 18 and Ubuntu 14 but not on Ubuntu 20 (all
LTS versions and all
with Git 2.29.2) so maybe something else is also at play here...

Cheers,

Philippe.