t0001,t1510,t3301: use sane_unset which always returns with status 0

t0001,t1510,t3301: use sane_unset which always returns with status 0

On some shells (like /usr/xpg4/bin/sh on Solaris), unset will exit
non-zero when passed the name of a variable that has not been set.  Use
sane_unset instead so that the return value of unset can be ignored while
the && linkage of the test script can be preserved.

Signed-off-by: Brandon Casey <redacted>
Signed-off-by: Junio C Hamano <redacted>