config: extract function to parse config pairs

config: extract function to parse config pairs

The function `git_config_parse_parameter` is responsible for parsing a
`foo.bar=baz`-formatted configuration key, sanitizing the key and then
processing it via the given callback function. Given that we're about to
add a second user which is going to process keys which already has keys
and values separated, this commit extracts a function
`config_parse_pair` which only does the sanitization and processing
part as a preparatory step.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>