fetch: lift up parsing of "fetch.output" config variable

fetch: lift up parsing of "fetch.output" config variable

Parsing the display format happens inside of `display_state_init()`. As
we only need to check for a simple config entry, this is a natural
location to put this code as it means that display-state logic is neatly
contained in a single location.

We're about to introduce a new "porcelain" output format though that is
intended to be parseable by machines, for example inside of a script.
This format can be enabled by passing the `--porcelain` switch to
git-fetch(1). As a consequence, we'll have to add a second callsite that
influences the output format, which will become awkward to handle.

Refactor the code such that callers are expected to pass the display
format that is to be used into `display_state_init()`. This allows us to
lift up the code into the main function, where we can then hook it into
command line options parser in a follow-up commit.

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