windows: gracefully handle when the username cannot be determined
This assumes implementation details, but I don't see any other way than to check
the environment variables ourselves (which would miss out on any future
enhancements that Python may make). This was originally reported as
https://foss.heptapod.net/mercurial/tortoisehg/thg/-/issues/5835.
#require symlink
https://bz.mercurial-scm.org/1438
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch