completion: list variable names reliably with 'git config --name-only'

Recenty I created a multi-line branch description with '.' and '=' characters on one of the lines, and noticed that fragments of that line show up when completing set variable names for 'git config', e.g.: $ git config --get branch.b.description Branch description to fool the completion script with a second line containing dot . and equals = characters. $ git config --unset <TAB> ... second line containing dot . and equals ... The completion script runs 'git config --list' and processes its output to strip the values and keep only the variable names. It does so by looking for lines containing '.' and '=' and outputting everything before the '=', which was fooled by my multi-line branch description. A similar issue exists with aliases and pretty format aliases with multi-line values, but in that case 'git config --get-regexp' is run and lines in its output are simply stripped after the first space, so subsequent lines don't even have to contain '.' and '=' to fool the completion script. Use the new '--name-only' option added in the previous commit to list config variable names reliably in both cases, without error-prone post processing. Signed-off-by: SZEDER Gábor <szeder@ira.uka.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>