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.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a