censor: migrate the logic to a set of `censor_revs`

Instead of considering a special unique censored revision within the code, we
consider a set of revision (currently always of size 1). This make the main code
less censor-centric and prepare for the usage of a similar approach for
stripping changesets.

Differential Revision: https://phab.mercurial-scm.org/D10903

mercurial/revlogutils/rewrite.py

@@ -146,20 +146,22 b' def v2_censor(revlog, tr, censornode, to'
     old_index = revlog.index
     docket = revlog._docket
 
-    censor_rev = revlog.rev(censornode)
+    censor_revs = {revlog.rev(censornode)}
     tombstone = storageutil.packmeta({b'censored': tombstone}, b'')
 
-    censored_entry = revlog.index[censor_rev]
-    index_cutoff = revlog.index.entry_size * censor_rev
-    data_cutoff = censored_entry[ENTRY_DATA_OFFSET] >> 16
-    sidedata_cutoff = revlog.sidedata_cut_off(censor_rev)
+    first_excl_rev = min(censor_revs)
+
+    first_excl_entry = revlog.index[first_excl_rev]
+    index_cutoff = revlog.index.entry_size * first_excl_rev
+    data_cutoff = first_excl_entry[ENTRY_DATA_OFFSET] >> 16
+    sidedata_cutoff = revlog.sidedata_cut_off(first_excl_rev)
 
     with pycompat.unnamedtempfile(mode=b"w+b") as tmp_storage:
         # rev → (new_base, data_start, data_end, compression_mode)
         rewritten_entries = _precompute_rewritten_delta(
             revlog,
             old_index,
-            {censor_rev},
+            censor_revs,
             tmp_storage,
         )
 
@@ -182,16 +184,18 b' def v2_censor(revlog, tr, censornode, to'
             ) = open_files
 
             # writing the censored revision
+
+            # Writing all subsequent revisions
+            for rev in range(first_excl_rev, len(old_index)):
+                if rev in censor_revs:
             _rewrite_censor(
                 revlog,
                 old_index,
                 open_files,
-                censor_rev,
+                        rev,
                 tombstone,
             )
-
-            # Writing all subsequent revisions
-            for rev in range(censor_rev + 1, len(old_index)):
+                else:
                 _rewrite_simple(
                     revlog,
                     old_index,