emitrevision: simplify the fallback to computed delta

Not using the stored delta, or having a full snapshot on disk behave the same
ways, so lets use the same code path for that, this is simpler, and it update
will be simpler.

mercurial/utils/storageutil.py

@@ -430,24 +430,16 b' def emitrevisions('
         # There is a delta in storage. We try to use that because it
         # amounts to effectively copying data from storage and is
         # therefore the fastest.
-        elif deltaparentrev != nullrev:
-            # If the stored delta works, let us use it !
-            if is_usable_base(deltaparentrev):
+        elif is_usable_base(deltaparentrev):
                 baserev = deltaparentrev
-            # No guarantee the receiver has the delta parent. Send delta
-            # against last revision (if possible), which in the common case
-            # should be similar enough to this revision that the delta is
-            # reasonable.
-            elif prevrev is not None:
-                baserev = prevrev
             else:
-                baserev = nullrev
-
-        # Storage has a fulltext revision.
-
-        # Let's use the previous revision, which is as good a guess as any.
-        # There is definitely room to improve this logic.
-        elif prevrev is not None:
+            # No guarantee the receiver has the delta parent, or Storage has a
+            # fulltext revision.
+            #
+            # Send delta against last revision (if possible), which in the
+            # common case should be similar enough to this revision that the
+            # delta is reasonable.
+            if prevrev is not None:
             baserev = prevrev
         else:
             baserev = nullrev