lfs: prefetch lfs blobs when applying merge updates In addition to merge, this method ultimately gets called by many commands: - backout - bisect - clone - fetch - graft - import (without --bypass) - pull -u - rebase - strip - share - transplant - unbundle - update Additionally, it's also called by histedit, shelve, unshelve, and split, but it seems that the related blobs should always be available locally for these. For `hg update`, it happens after the normal argument checking and pre-update hook processing, and remote corruption is detected prior to manipulating the working directory. Other commands could use this treatment (archive, cat, revert, etc), but this covers so many of the frequently used bulk commands, it seems like a good starting point. Losing the verbose message that prints the file name before a corrupt blob aborts the command is a little sad, because there's no easy way to go from oid to file name. I'd like to change that message to list the file name so it looks cleaner and less cryptic, but the pointer object is nowhere near where it needs to be to do this. So punt on that for now.

lfs: factor out a method for extracting the pointer of a single file This will be useful for filesets, among other things, instead of traversing the whole context.

py3: add b'' to literals in check-config.py # skip-blame because we are just adding b'' Differential Revision: https://phab.mercurial-scm.org/D2046

check-config: specify the mode 'rb' to open the file Differential Revision: https://phab.mercurial-scm.org/D2045

py3: use open() instead of file() file() is not present in Python 3 Differential Revision: https://phab.mercurial-scm.org/D2044

py3: use pycompat.strkwargs() to convert kwargs' key to str Differential Revision: https://phab.mercurial-scm.org/D2043