src/bin/duffman: Handle errors correctly

src/bin/duffman: Handle errors correctly

In src/bin/duffman line 30:
cd $(git rev-parse --show-toplevel);
   ^-- SC2046 (warning): Quote this to prevent word splitting.
     ^---------------------------^ SC2312 (info): Consider invoking this command separately to avoid masking its return value (or use '|| true' to ignore).

Reported-by: shellcheck(1)
Signed-off-by: Alejandro Colomar <alx@kernel.org>