RevWalk: Don't release during inMergeBase()

In bc1af8459e ("RevWalk: Don't reset ObjectReader when stopping") we
stopped releasing the reader when the current log traversal is over.
This should have also been applied to the merge base logic that is
buried within MergeGenerator, but got missed.

Change-Id: I8328f43f02cba06fd545e22134872e781b9d4d36
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/MergeBaseGenerator.java

@@ -138,7 +138,6 @@ class MergeBaseGenerator extends Generator {
 			final RevCommit c = pending.next();
 			if (c == null) {
 				walker.reader.walkAdviceEnd();
-				walker.reader.release();
 				return null;
 			}