coverage erase doesn't erase coverage when created with -p

[nedbat]

Originally reported by Lasse Schuirmann (Bitbucket: sils1297, GitHub: sils1297)

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Steps to reproduce:

#!bash
> git clone https://github.com/coala-analyzer/coala/
> cd coala
> ./run_tests.sh -c  # Press Ctrl+C before test suite ends so it doesn't get combined
...
> ls -la .cov*
-rw-r--r-- 1 lasse lasse  9642 Jun 29 15:36 .coverage.lssteadarch.31498.221346
-rw-r--r-- 1 lasse lasse 14017 Jun 29 15:36 .coverage.lssteadarch.31500.940577
-rw-r--r-- 1 lasse lasse  7698 Jun 29 15:36 .coverage.lssteadarch.31573.919492
-rw-r--r-- 1 lasse lasse  7647 Jun 29 15:36 .coverage.lssteadarch.31577.307965
> coverage erase
> ls -la .cov*
-rw-r--r-- 1 lasse lasse  9642 Jun 29 15:36 .coverage.lssteadarch.31498.221346
-rw-r--r-- 1 lasse lasse 14017 Jun 29 15:36 .coverage.lssteadarch.31500.940577
-rw-r--r-- 1 lasse lasse  7698 Jun 29 15:36 .coverage.lssteadarch.31573.919492
-rw-r--r-- 1 lasse lasse  7647 Jun 29 15:36 .coverage.lssteadarch.31577.307965

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/379

[nedbat]

Duplicate of #262.