TW-88158: remove MercurialCommitsInfoBuilderSupport and CommitsAndMountPointsCommand together with the custom hg extension. - The extension point was introduced for Upsource and is no longer needed. - The hg extension was not compatible with python 3 and led to test errors. (grafted from cb5e6f9a2bcd450e660c89305b190f5ed272e0d6)

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<project version="4">
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