Problem: `vim.pack.update()` doesn't update source's default branch if
it has changed on the remote. It can be done by executing
`git remote set-head origin --auto` for every plugin on every update,
but it feels like too much extra work (which requires Internet
connection) for a very rare use case.
This matters since `version = nil` will keep pointing to previous
default branch even after `vim.pack.update()`.
Solution: Document that in order for `version = nil` to point to the
source's new default branch, perform clean re-install.
e6d955cb2c