[sheets] fix splitcell to handle attribute/text pairs

[ajkerrigan]

Have splitcell account for receiving a list of (attr, text) tuples rather than a list of text lines.

Return a list of (attr, line) for each wrapped line of text. The intent is to have _splitlines() play nicely with the displayer_generic() changes here.

Open questions:

• Are there cases where _splitcell() will still get a list of strings rather than a list of (attr, text) pairs?
• Is there a better/cleaner way to handle this?
• Is there a better place to handle this?

Closes #2019

Unrelated drive-by fix: Looks like we were trying to sum a generator here when counting deferred changes during a commit-sheet command, and that was blowing up vgit tests. Noooope, let #2009 handle that.

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[anjakefala]

Btw, the vgit test fix is here! #2009

The reason summing the generator blows up is because sum = aggregators::vsum.

[ajkerrigan]

Btw, the vgit test fix is here! #2009

The reason summing the generator blows up is because sum = aggregators::vsum.

Ahhh thanks! I didn't see anything that looked obviously related, I'll remove the drive-by piece of this then 🙈

[saulpw]

This looks good to me, @ajkerrigan! Thanks for taking care of it.