git-rebase: document suppression of duplicate commits

git-rebase uses format-patch's --ignore-if-in-upstream
option, but we never document the user-visible behavior. The
example is placed near the top of the example list rather
than at the bottom because it is:
  a. a simple example
  b. a reasonably common scenario for many projects (mail
     some patches which get accepted upstream, then rebase)

[sp: Corrected direction of 'HEAD..<upstream>' set comparsion]

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

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