debug-revlog: keep the revlog open for the analysis duration

This is more explicit.  No performance differences were observed.

mercurial/revlogutils/debug.py

@@ -346,6 +346,7 b' def debug_revlog(ui, revlog):'
             l[1] = size
         l[2] += size
 
+    with r.reading():
     numrevs = len(r)
     for rev in range(numrevs):
         p1, p2 = r.parentrevs(rev)
@@ -371,7 +372,9 b' def debug_revlog(ui, revlog):'
                 addsize(size, snapsizedepth[0])
         else:
             nad = (
-                delta != p1 and delta != p2 and not r.isancestorrev(delta, rev)
+                    delta != p1
+                    and delta != p2
+                    and not r.isancestorrev(delta, rev)
             )
             chainlengths.append(chainlengths[delta] + 1)
             baseaddr = chainbases[delta]