recipes: use tags in a nested git repo

According to https://help.launchpad.net/Packaging/SourceBuilds/Recipes#Specifying_revisions, "For git, a revision specifier may be anything that you could pass to git rev-parse in a clone of the given repository. ".
A working git command for tag 1.6.4 would be:
git rev-parse 1.6.4
But the following line:
nest vendor lp:~sajoupa/telegraf/+git/vendor 1.6.4 vendor
is interpreted by LP as a subdir, not an argument to rev-parse:
subprocess.CalledProcessError: Command '['git', '-C', '/home/buildd/build-RECIPEBRANCHBUILD-1596142/chroot-autobuild/home/buildd/work/tree/recipe/1.6.4', 'rev-parse', 'vendor']' returned non-zero exit status 128

https://launchpadlibrarian.net/375110198/buildlog.txt.gz

I tried various different syntaxes, but they all failed for the same reason.

Or maybe there is a way to specify tags that I didn't find ? (in which case, adding it to https://help.launchpad.net/Packaging/SourceBuilds/Recipes#Specifying_revisions would be great).

Thanks.