mergetool--lib: improve support for vimdiff-style tool variants

mergetool--lib: improve support for vimdiff-style tool variants

The merge tools vimdiff2, vimdiff3, gvimdiff2, gvimdiff3 and bc3 are all
variants of the main tools vimdiff and bc. They are implemented in the
main and a one-liner script that just sources it exist for each.

Allow variants ending in [0-9] to be correctly wired without the need
for such one-liners, so instead of 5 scripts, only 1 (gvimdiff) is
needed.

Signed-off-by: pudinha <redacted>
Signed-off-by: Junio C Hamano <redacted>