RevFlagSetTest: Fix compilation error flagged by error prone

RevFlagSetTest: Fix compilation error flagged by error prone

This fixes error flagged by error prone:

Java compilation in rule '//org.eclipse.jgit.test:jgit' failed: Worker
process sent response with exit code: 1.
org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevFlagSetTest.java:149:
error: [CollectionIncompatibleType] Argument '"bob"' should not be
passed to this method; its type String is not compatible with its
collection's type argument RevFlag
		assertFalse(set.contains("bob"));

Change-Id: I4a971ce92fee55e28b2ab0c7b716ac20fa9c6709
Signed-off-by: David Ostrovsky <david@ostrovsky.org>