Problem: Changing `src` of already installed plugin currently takes
effect immediately inside `vim.pack.add()` and acts as "delete and
later fresh install". Although more robust, this might lead to
unintentional data loss (since plugin is deleted) if the plugin was
manually modified or the new source is not valid.
Also this introduces unnecessary differentiation between "change
`version`" and "change `src`" of already installed plugin.
Solution: Require an explicit `vim.pack.update()` to change plugin's
source. It is done by conditionally changing `origin` remote of the
Git repo. The effect does not require update confirmation in order to
have new changes fetched from the new `src` right away.
If in the future there are more types of plugins supported (i.e. not
only Git repos), also do extra work (like delete + install) during
`vim.pack.update()`.
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