Switch RevWalker from release to close

Gerrit's JGit has been updated to a version without 'release'. Hence,
we update to using 'close' instead.

(After this commit, the code compiles again. Updating to AutoClosable
is done in follow-up commits, after the exception catching got cleaned
up a bit.)

Change-Id: I8910c314f01b09238c960c01af21c61890600674