config: report correct line number upon error

config: report correct line number upon error

When get_value() parses a key/value pair, it is possible that the line
number is decreased (because the \n has been consumed already) before the
key/value pair is passed to the callback function, to allow for the
correct line to be attributed in case of an error.

However, when git_parse_source() asks get_value() to parse the key/value
pair, the error reporting is performed *after* get_value() returns.

Which means that we have to be careful not to increase the line number
in get_value() after the callback function returned an error.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>