When git-subtree is pulling data using the tag reference,
it writes the tag's sha1 into a metadata.
It could be a problem next time, since this commit object
is not a part of main tree, and could be lost.
Steps to reproduce:
# add some stuff to a existing tree
# history will refer to v0.1 tag object instead of v0.1^{commit}
git subtree add --squash -P dir/ repo v0.1
# prune all dangling objects including external v0.1 tag
git gc --aggressive --prune=all
# this will fail
git subtree pull --squash -P dir/ repo v0.2
