deltas: skip if projected delta size is bigger than previous snapshot

Before computing any delta, we get a basic estimation of the delta size we can
expect and the resulted compressed value. We then checks this projected size
against the `size(snapshotⁿ) > size(snapshotⁿ⁺¹)` constraint. This allows to
exclude potential base candidates before doing any expensive computation.

This only apply to the intermediate-snapshot case since this constraint only
apply to them.

For some pathological cases of a private repository this step provide a
significant performance boost (timing from `hg perfrevlogwrite`):

before: 14.115908 seconds
after:   3.145906 seconds

mercurial/revlogutils/deltas.py

@@ -698,6 +698,12 b' def _candidategroups(revlog, textlen, p1'
                         # delta lower bound is larger than accepted upper bound
                         continue
 
+                    # check the relative constraint on the delta size
+                    revlength = revlog.length(rev)
+                    if revlength < lowestrealisticdeltalen:
+                        # delta probable lower bound is larger than target base
+                        continue
+
             group.append(rev)
         if group:
             # XXX: in the sparse revlog case, group can become large,