checkout: pass program-readable token to unified "main"

checkout: pass program-readable token to unified "main"

The "git checkout", "git switch", and "git restore" commands share a
single implementation, checkout_main(), which switches error message
it gives using the usage string passed by each of these three
front-ends.

In order to be able to tweak behaviours of the commands based on
which one we are executing, invent an enum that denotes which one of
these three commands is currently executing, and pass that to
checkout_main() instead.  With this step, there is no externally
visible behaviour change, as this enum parameter is only used to
choose among the three usage strings.

Signed-off-by: Junio C Hamano <gitster@pobox.com>