reset.c: replace switch by if-else

reset.c: replace switch by if-else

The switch statement towards the end of reset.c is missing case arms
for KEEP and MERGE for no obvious reason, and soon the only non-empty
case arm will be the one for HARD. So let's proactively replace it by
if-else, which will let us move one if statement out without leaving
funny-looking left-overs.

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>