completion: add a helper function to get config variables

Currently there are a few completion functions that perform similar 'git
config' queries and filtering to get config variable names: the completion
of pretty aliases, aliases, and remote groups for 'git remote update'.

Unify those 'git config' queries in a helper function to eliminate code
duplication.

Though the helper functions to get pretty aliases and alieses are reduced
to mere one-liner wrappers around the newly added function, keep these
helpers still, because users' completion functions out there might depend
on them.  And they keep their callers a tad easier to read, too.

Add tests for the pretty alias and alias helper to show that they work
as before; not for the remote groups query, though, because that's not
extracted into a helper function and it's not worth the effort to do so
for a sole callsite.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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