bash-completion: Future-proof --help-*-list "cmake version" filtering

A future version of CMake may not print the "cmake version" line at the
beginning of the --help-*-list output.  Filter out the line with 'grep'
instead of 'tail' to tolerate output from versions of CMake with and
without the version line.
Merge topic 'bash-completion-future-filter' into next

c5ee80b bash-completion: Future-proof --help-*-list "cmake version" filtering