largefiles: fix a traceback in lfconvert if a largefile is missing (
issue3519)
The largefile may be missing for various reasons, including that a remote
repository was cloned without the --all-largefiles option. Therefore, it seems
reasonable to attempt to download the missing files and failing that, abort and
indicate the affected file and revision so the user can manually fix the
problem.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda
$ cd ..