delta-find: stop using heuristic to determine if we are creating a snapshot

This avoid assuming a changeset is a snapshot when it is actually something
simpler.

mercurial/revlogutils/deltas.py

@@ -1067,7 +1067,7 b' class _DeltaSearch(_BaseDeltaSearch):'
             and good not in (self.p1, self.p2)
             and self.revlog.issnapshot(good)
         ):
-            self.current_stage = _STAGE_SNAPSHOT
+            assert self.current_stage == _STAGE_SNAPSHOT
             # refine snapshot down
             previous = None
             while previous != good:
@@ -1336,7 +1336,9 b' class deltacomputer:'
 
         return delta
 
-    def _builddeltainfo(self, revinfo, base, target_rev=None):
+    def _builddeltainfo(
+        self, revinfo, base, target_rev=None, as_snapshot=False
+    ):
         # can we use the cached delta?
         revlog = self.revlog
         chainbase = revlog.chainbase(base)
@@ -1354,7 +1356,8 b' class deltacomputer:'
         snapshotdepth = None
         if revlog.delta_config.sparse_revlog and deltabase == nullrev:
             snapshotdepth = 0
-        elif revlog.delta_config.sparse_revlog and revlog.issnapshot(deltabase):
+        elif revlog.delta_config.sparse_revlog and as_snapshot:
+            assert revlog.issnapshot(deltabase)
             # A delta chain should always be one full snapshot,
             # zero or more semi-snapshots, and zero or more deltas
             p1, p2 = revlog.rev(revinfo.p1), revlog.rev(revinfo.p2)
@@ -1679,6 +1682,7 b' class deltacomputer:'
                     revinfo,
                     candidaterev,
                     target_rev=target_rev,
+                    as_snapshot=search.current_stage == _STAGE_SNAPSHOT,
                 )
                 if self._debug_search:
                     delta_end = util.timer()

tests/test-sparse-revlog.t

@@ -279,7 +279,7 b' Test `debug-delta-find`'
   DBG-DELTAS-SEARCH:     uncompressed-delta-size=33050
   DBG-DELTAS-SEARCH:     delta-search-time=* (glob)
   DBG-DELTAS-SEARCH:     DELTA: length=19179 (GOOD)
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=1 - search-rounds=1 try-count=1 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=1 - search-rounds=1 try-count=1 - delta-type=delta  snap-depth=-1 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
   $ hg debug-delta-find SPARSE-REVLOG-TEST-FILE 4971 --source p1
   DBG-DELTAS-SEARCH: SEARCH rev=4971
   DBG-DELTAS-SEARCH: ROUND #1 - 3 candidates - search-down