bug#19899: sed: fix mishandling of overlapping address ranges

[Jim Meyering]

[resending, to @debbugs.gnu.org, not @bugs.gnu.org]

Thank you Ethan for the report, and Norihiro for the patch.
I've made adjustments to the patch, primarily to use the
init.sh-based style of test case (permitting to add just one
file for each test case, rather than 3 or more) and rewriting
the commit log text and NEWS entry.

Norihiro, please sanity-check before I push this.

0001-sed-fix-mishandling-of-overlapping-address-ranges.patch
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