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>

repo

@@ -149,7 +149,7 @@ if not REPO_REV:
 BUG_URL = 'https://bugs.chromium.org/p/gerrit/issues/entry?template=Repo+tool+issue'
 
 # increment this whenever we make important changes to this script
-VERSION = (2, 17)
+VERSION = (2, 21)
 
 # increment this if the MAINTAINER_KEYS block is modified
 KEYRING_VERSION = (2, 3)
@@ -382,6 +382,11 @@ def InitParser(parser, gitc_init=False):
   group.add_option('--no-clone-bundle',
                    dest='clone_bundle', action='store_false',
                    help='disable use of /clone.bundle on HTTP/HTTPS (default if --partial-clone)')
+  group.add_option('--git-lfs', action='store_true',
+                   help='enable Git LFS support')
+  group.add_option('--no-git-lfs',
+                   dest='git_lfs', action='store_false',
+                   help='disable Git LFS support')
 
   # Tool.
   group = parser.add_option_group('repo Version options')