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While GHC's own aclocal.m4 is generated by the aclocal tool, other packages' aclocal.m4 are committed in the repository. Previously `.gitignore` included an entry which covered *any* file named `aclocal.m4`, which lead to quite some confusion (e.g. see #21740). Fix this by modifying GHC's `.gitignore` to only cover GHC's own `aclocal.m4`.
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