lfs: use the local store method for opening a blob
I noticed that when I cloned without updating and then turned around and pushed
that clone to an lfs server, it was only trying to find the blob in the local
store.
Writes to the dummyremote (file based store) use local.read(), which looks at
both the usercache and local store.
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).