git config: trivial rename in preparation for parseopt

git config: trivial rename in preparation for parseopt

Essentially this replaces 'file' with 'prefix' in the cases where the
variable is used as a prefix, which is consistent with other git
commands.

When using the --list option general errors where not properly reported,
only errors related with the 'file'. Now they are reported, and 'file'
is irrelevant.

That reduces the rest of 'file' usage to nothing, therefore now only
'prefix' remains.

Suggested by Johannes Schindelin.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>