branchcache: do not copy the `_dirty` flag
If the inherited branch cache is dirty, it will be written on disk, and the
super-set did not need to modify it, the on disk value for the subset will be
re-useable as is. So the super set does not needs to write the very same content
itself.
$ 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 ..