verify: keep the revlog open for reading while verifying it

This is more explicit. No performances difference were observed.

mercurial/verify.py

@@ -270,6 +270,7 class verifier:
         progress = ui.makeprogress(
             _(b'checking'), unit=_(b'changesets'), total=len(repo)
         )
+        with cl.reading():
         for i in repo:
             progress.update(i)
             n = cl.node(i)