Previously, `:PlugDiff` would show every new commit to a plugin's git
repo. This makes sense for the general case, but makes less sense when a
plugin lives in a subdirectory of the repo (and is configured with the
'rtp' option). This makes it difficult to determine which commits relate
to the plugin and which are unrelated.
This changes `:PlugDiff` to filter out any commits outside of the 'rtp'
folder.
Some consequences:
* This does not change the `:PlugUpdate` UI. This means `:PlugUpdate`
may pull down non-plugin commits, display that it has updated the
plugin, and then `:PlugDiff` will show no updates (since such commits
fall out of the 'rtp' path).
* It also means there's no UI to revert non-plugin updates, as they
don't show up in `:PlugDiff`.
Test: Manual on Linux machine
Nate Fischer