debugrevlog: fix average size computation for empty data (issue6167)

If the file has no full snapshot (eg: was always empty), `hg debugrevlog` would
fails when trying to compute their average size.

mercurial/debugcommands.py

@@ -2274,6 +2274,9 b' def debugrevlog(ui, repo, file_=None, **'
     totalrawsize = datasize[2]
     datasize[2] /= numrevs
     fulltotal = fullsize[2]
+    if numfull == 0:
+        fullsize[2] = 0
+    else:
     fullsize[2] /= numfull
     semitotal = semisize[2]
     snaptotal = {}