overlayworkingctx: rename misleadingly named `isempty()` method
This method is only about whether there are file changes, not about whether the
commit will be empty or not.
One user of the method was incorrectly assuming the latter meaning, leading to
the bug for which a test case was added in D8727. I’ve added a FIXME to the
code.
The original motivation for the rename was that I want to add
`committablectx.isempty()`, that properly checks if a commit will be empty,
using the exact same logic as in `repo.commit()`, and I wanted to avoid a name
clash.
Differential Revision: https://phab.mercurial-scm.org/D8728
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