This PR makes it so that `open_diff_since`, as used by the branch diff,
is able to return a stable diff entity instead of creating a new one
every time this is called. In particular, the base text buffer for this
diff is now stable across branch diff refreshes, making it usable with
the side-by-side view. The strategy for keeping the diff entities alive
is the same one that we use for the uncommitted and unstaged diffs--the
`GitStore` only holds a weak pointer to each "since" diff, so when these
diffs are no longer in use by the branch diff multibuffer they can be
cleaned up.

Release Notes:

- N/A

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Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>