mergetools/meld: Use --help output to detect --output support
In v1.7.7-rc0~3^2 (2011-08-19), git mergetool's "meld" support learned
to use the --output option when calling versions of meld that are
detected to support it (1.5.0 and newer, hopefully).
Alas, it misdetects old versions (before 1.1.5, 2006-06-11) of meld as
supporting the option, so on systems with such meld, instead of
getting a nice merge helper, the operator gets a dialog box with the
text "Wrong number of arguments (Got 5)". (Version 1.1.5 is when meld
switched to using optparse. One consequence of that change was that
errors in usage are detected and signalled through the exit status
even when --help was passed.)
Luckily there is a simpler check that is more reliable: the usage
string printed by "meld --help" reliably reflects whether --output is
supported in a given version. Use it.