Add undo / redo icons

[gavinmn]

closes https://github.com/github/primer/issues/2163

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Latest commit: b471d50

The changes in this PR will be included in the next version bump.

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@primer/octicons	Minor

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[colebemis]

How do you feel about adding a little more space between the tail and the arrowhead?

Here are some under/redo icons I designed for Primer Prism that demonstrate what I mean:

[iansan5653]

Purely opinionated thought: Do you think maybe they are visually weighted too much toward the arrowhead? To me they kind of feel like they look like they are about to tip over, especially with a larger gap 😄. Maybe a smaller arrowhead and longer tail that goes all the way to the left?

[gavinmn]

Just pushed these changes:

I widened the radius of the turn to get some more separation between the end of the arrow and the tail. The bottom preview included is an example of the tail going further across. I feel like this causes the icon to loose the "swoop" feeling that gives the "undo" feeling and it ends up looking more like "u turn" to me.

[colebemis]

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