largefiles: prevent committing a missing largefile Previously, if the largefile was deleted at the time of a commit, the standin was silently not updated and its current state (possibly garbage) was recorded. The test makes it look like this is somewhat of an edge case, but the same thing happens when an `hg revert` followed by `rm` changes the standin. Aside from the second invocation of this in lfutil.updatestandinsbymatch() (which is what triggers this test case), the three other uses are guarded by dirstate checks for added or modified, or an existence check in the filesystem. So aborting in lfutil.updatestandins() should be safe, and will avoid silent skips in the future if this is used elsewhere.