typing: add trivial type hints to the convert extension's common modules
This started as ensuring that the `encoding` and `orig_encoding` attributes has
a type other than `Any`, so pytype can catch problems where it needs to be str
for stdlib encoding and decoding. It turns out that adding the hint in
`mercurial.encoding` is what was needed, but I picked a bunch of low hanging
fruit while here. There's definitely more to do, and I see a problem where
`shlex.shlex` is being fed bytes instead of str, but there are not enough type
hints yet to make pytype notice.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
Ensure that the username access fails gracefully if assumptions about the
environment made by python do not hold.
#if windows
>>> import os
>>> from mercurial import util
>>> os.environ.pop('LOGNAME', None) and None
>>> os.environ.pop('USER', None) and None
>>> os.environ.pop('LNAME', None) and None
>>> os.environ.pop('USERNAME', None) and None
>>> print(util.username())
None
#endif
$ cd ..