Return false from MergeUtil.canMerge on LargeObjectException

JGit's merge code depends on ObjectLoader.getCachedBytes(), which may
throw LargeObjectException, in which case it can't attempt a content
merge. For non-content merges (no difference from one parent), we
should not hit this case.

Change-Id: I8c4c42d8cd2234e6d3b8e38d9c08b8b89e82e37b