reset: modernize flags passed to `reset_working_tree()`

reset: modernize flags passed to `reset_working_tree()`

The flags passed to `reset_working_tree()` are declared as defines. This
has fallen a bit out of practice nowadays, where we instead prefer to
use enums. Furthermore, the prefix of those flags does not match the
function name anymore after the rename in the preceding commit.

Adapt the code to follow modern best practices and adapt the flag names.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>