misc/emacs: Detect stale coverage reports
misc/emacs: Detect stale coverage reports

When the coverage report file is older than the file we're
showing the coverage report for, then we show a simple message
to state this fact.

R=adonovan, dominik.honnef, bradfitz
CC=golang-dev
https://codereview.appspot.com/12919044

Committer: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

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