The command `hg up -C` by default moves to the latest revision on the
current branch. The `discard` function was trying to update to a
different branch, in case it was provided, by passing a `-r REVISION`
argument. Not only is this not the intended effect of the `discard`
function, but this also could update to a different branch that hasn't
been pulled yet, which is how we were experiencing trouble.
Instead, we unconditionally do `hg up -C -r .` to "update" to the
current revision (i.e. to "."), while `-C/--clean`ing the current
directory. This is similar to `hg revert --all`, except that it also
undoes the merge state of the working directory, in case there was
any.
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