completion: use "prev" variable instead of introducing "prevword"

completion: use "prev" variable instead of introducing "prevword"

In both _git_checkout and _git_switch a new "prevword" variable were
introduced, however the "prev" variable already contains the last word.

The "prevword" variable is replaced with "prev", and the case is moved
to the beginning of the function, like it's done in many other places
(e.g. _git_commit). Also the indentaion of the case is fixed.

Signed-off-by: Ákos Uzonyi <redacted>
Signed-off-by: Junio C Hamano <redacted>