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

If we have a delta, we check constraints against a lower bound estimate of the
resulting compressed delta. 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
further performance boost (timing from `hg perfrevlogwrite`):

before:  3.010646 seconds
after:   2.609307 seconds

mercurial/revlogutils/deltas.py

@@ -966,6 +966,8 b' class deltacomputer(object):'
             snapshotlimit = revinfo.textlen >> snapshotdepth
             if snapshotlimit < lowestrealisticdeltalen:
                 return None
+            if revlog.length(base) < lowestrealisticdeltalen:
+                return None
         header, data = revlog.compress(delta)
         deltalen = len(header) + len(data)
         offset = revlog.end(len(revlog) - 1)