tests: run check-code with warnings and maintain a whitelist
check-code has fine warning checks, but they are a bit noisy and nobody used
them.
Now the warnings will be run in the test suite, where a list of accepted
warnings will be maintained.
Those who introduce or touch a line with a warning will now have to update the
whitelist ... or fix the warning.
test that 'hg commit' does not crash if the user removes a newly added file
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ touch b
$ hg add b
$ rm b
$ hg commit -A -m"comment #1"
removing b
nothing changed
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