debugrevlog: handle numrevs == numfull case (issue3537)

Instead of tracing back with a ZeroDivisionError.

tests/test-debugcommands.t

@@ -0,0 +1,25 b''
+  $ hg init debugrevlog
+  $ cd debugrevlog
+  $ echo a > a
+  $ hg ci -Am adda
+  adding a
+  $ hg debugrevlog -m
+  format : 1
+  flags  : inline
+  
+  revisions     :  1
+      merges    :  0 ( 0.00%)
+      normal    :  1 (100.00%)
+  revisions     :  1
+      full      :  1 (100.00%)
+      deltas    :  0 ( 0.00%)
+  revision size : 44
+      full      : 44 (100.00%)
+      deltas    :  0 ( 0.00%)
+  
+  avg chain length  : 0
+  compression ratio : 0
+  
+  uncompressed data size (min/max/avg) : 43 / 43 / 43
+  full revision size (min/max/avg)     : 44 / 44 / 44
+  delta size (min/max/avg)             : 0 / 0 / 0

mercurial/commands.py

@@ -2245,6 +2245,11 b' def debugrevlog(ui, repo, file_ = None, '
             elif delta != nullrev:
                 numother += 1
 
+    # Adjust size min value for empty cases
+    for size in (datasize, fullsize, deltasize):
+        if size[0] is None:
+            size[0] = 0
+
     numdeltas = numrevs - numfull
     numoprev = numprev - nump1prev - nump2prev
     totalrawsize = datasize[2]
@@ -2252,7 +2257,8 b' def debugrevlog(ui, repo, file_ = None, '
     fulltotal = fullsize[2]
     fullsize[2] /= numfull
     deltatotal = deltasize[2]
-    deltasize[2] /= numrevs - numfull
+    if numrevs - numfull > 0:
+        deltasize[2] /= numrevs - numfull
     totalsize = fulltotal + deltatotal
     avgchainlen = sum(chainlengths) / numrevs
     compratio = totalrawsize / totalsize