revlog: filter out "invalid" delta base candidates
There are bases that we know won't produce a good delta chain. For example, if
the base is already bigger than twice the size of the text we store, we know
the resulting delta chain will never be valid.
We might make the check a bit more powerful and generic in the future, but
this looks a good start.
In particular, empty file (size 0) will never find a good base, so we should
stop spending time trying to find one.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).