fetch: add new config option fetch.all

fetch: add new config option fetch.all

Introduce a boolean configuration option fetch.all which allows to
fetch all available remotes by default. The config option can be
overridden by explicitly specifying a remote or by using --no-all.
The behavior for --all is unchanged and calling git-fetch with --all
and a remote will still result in an error.

Additionally, describe the configuration variable in the config
documentation and implement new tests to cover the expected behavior.
Also add --no-all to the command-line documentation of git-fetch.

Signed-off-by: Tamino Bauknecht <dev@tb6.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>