Refactor calculation of "other regressions" output.

Refactor calculation of "other regressions" output, calculate confidence in these local regressions, and only output if high confidence. Previously, we output every change if it was greater than some small %, which resulted in a lot of noise.

Example output:

Other regressions may have occurred:
   Depot                                          Range                                         Confidence
  chromium  fc062604f35d88e607e36ae5f91d19d7cb2251a2..400664c4101576b48e30da67d57ecffc4d83fcb0    95%    

BUG=319898
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=243246

[tonyg, 2014-01-06 20:16:07]

lgtm

Very nice!

[commit-bot: I haz the power, 2014-01-06 21:11:48]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/simonhatch@chromium.org/122563003/50001

[commit-bot: I haz the power, 2014-01-07 10:47:16]

Change committed as 243246