init: add an option --enable-git-lfs-filter

It was reported that git-lfs did not work with git-repo. Specifically,
`git read-tree -u` run by `repo sync` would fail git-lfs's smudge
filter. See https://github.com/github/git-lfs/issues/1422.

In fact, by the time `git read-tree -u` is run, the repository is not
bare. It is just that, the working directory is not the same as the
.git directory. git-lfs's filter should work. No one seems to have
delved into that issue.

Today, with newer versions of git-repo and git-lfs, that issue will
not reproduce. Tested with
- git 2.33, git-lfs 2.13 on macOS
- git 2.17, git-lfs 2.3 on ubuntu

So, it seems fine to add an option --enable-git-lfs-filter, default to
false, and stat that it may not work with older versions of git and
git-lfs in the help doc.

Bug: https://crbug.com/gerrit/14516
Change-Id: I8d21854eeeea541e072f63d6b10ad1253b1a9826
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/328359
Tested-by: XD Trol <milestonejxd@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/init.py

@@ -291,6 +291,15 @@ to update the working directory files.
     if opt.submodules:
       m.config.SetBoolean('repo.submodules', opt.submodules)
 
+    if opt.git_lfs is not None:
+      if opt.git_lfs:
+        git_require((2, 17, 0), fail=True, msg='Git LFS support')
+
+      m.config.SetBoolean('repo.git-lfs', opt.git_lfs)
+      if not is_new:
+        print('warning: Changing --git-lfs settings will only affect new project checkouts.\n'
+              '         Existing projects will require manual updates.\n', file=sys.stderr)
+
     if opt.use_superproject is not None:
       m.config.SetBoolean('repo.superproject', opt.use_superproject)