config: add a repo_config_get_uint() helper

config: add a repo_config_get_uint() helper

Next commits add a 'hook.jobs' config option of type 'unsigned int',
so add a helper to parse it since the API only supports int and ulong.

An alternative is to make 'hook.jobs' an 'int' or parse it as an 'int'
then cast it to unsigned, however it's better to use proper helpers for
the type. Using 'ulong' is another option which already has helpers, but
it's a bit excessive in size for just the jobs number.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>