completion: complete configuration sections and variable names for 'git -c'

completion: complete configuration sections and variable names for 'git -c'

'git config' expects a configuration variable's name and value in
separate arguments, so we let the __gitcomp() helper append a space
character to each variable name by default, like we do for most other
things (--options, refs, paths, etc.).  'git -c', however, expects
them in a single option joined by a '=' character, i.e.
'section.name=value', so we should append a '=' character to each
fully completed variable name, but no space, so the user can continue
typing the value right away.

Add an option to the __git_complete_config_variable_name() function to
allow callers to specify an alternate suffix to add, and use it to
append that '=' character to configuration variables.  Update the
__gitcomp() helper function to not append a trailing space to any
completion words ending with a '=', not just to those option with a
stuck argument.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>