If a user aliased "hg log" to "hg log --graph" [^1] (apparently this is
widespread, see [^2]), mercurial based setups do not work, because "hg log"
output needs to be reset to a canonical form.

This is done setting the environment variable HGRCPATH= to an empty value.

A previous attempt at normalizing hg output was done on 2010-08-29, when
e0954e3 ("inline getoutput, set HGPLAIN to disable hg i18n"), introduced
HGPLAIN=1, but this change is not sufficient for protecting against `hg log`
aliasing.

An example of command that is fixed with this change is the compilation of
mercurial itself in local mode (here shown for 7.2.1):

```
  $ make local
  ...
  File "<HOME>/.cache/uv/builds-v0/.tmpcFxMQB/lib64/python3.14/site-packages/vcs_versioning/_run_cmd.py", line 173, in run
    res.check_returncode()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.14/subprocess.py", line 508, in check_returncode
    raise CalledProcessError(self.returncode, self.args, self.stdout,
                             self.stderr)
subprocess.CalledProcessError: Command '['hg', 'log', '-r', '.', '-T', '{node}\n{tag}\n{date|shortdate}']' returned non-zero exit status 255.
```

[^1]: `hg log` can be aliased editing the contents of `hgrc`:
    ```
    [alias]
    log = log --graph
    ```

[^2]: https://foss.heptapod.net/mercurial/mercurial-devel/-/commit/604c086ddde6f40309c03ae7e5050c7c8d87e75a