This is needed for compatibility with an upcoming release of
MozillaBuild, which distributes Mercurial as a Python package, not as a
standalone Windows program. As a result, it introduces "hg" into $PATH,
which "which" will happily prefer as the "hg" binary. This upsets
subprocess. So, we explicitly prefer "hg.exe" over "hg".
We could accomplish the same thing by calling which.whichall() and
sorting results. But this is more code and IMO not worth the effort to
implement.
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