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>

manifest_xml.py

@@ -666,6 +666,10 @@ https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md
   def HasSubmodules(self):
     return self.manifestProject.config.GetBoolean('repo.submodules')
 
+  @property
+  def EnableGitLfs(self):
+    return self.manifestProject.config.GetBoolean('repo.git-lfs')
+
   def GetDefaultGroupsStr(self):
     """Returns the default group string for the platform."""
     return 'default,platform-' + platform.system().lower()