I have recently learned why this issue had seemed transient to me previously: the hg eol extension apparently causes hg to create all files that are not explicitly set to LF or BIN using CRLF on Windows.  Thus, this is what happens:

"""
P:\Python\cpython\Doc>rem In a fresh checkout made with the 'eol' extension disabled:

P:\Python\cpython\Doc>make.bat update
The system cannot find the batch label specified - update

P:\Python\cpython\Doc>cd ..

P:\Python\cpython>hg up null
0 files updated, 0 files merged, 3689 files removed, 0 files unresolved

P:\Python\cpython>hg --config extensions.eol= up default
3689 files updated, 0 files merged, 0 files removed, 0 files unresolved

P:\Python\cpython>cd Doc

P:\Python\cpython\Doc>make.bat update
Updating 'tools\sphinx':
At revision 89001.
Updating 'tools\docutils':
At revision 89001.
Updating 'tools\jinja2':
At revision 89001.
Updating 'tools\pygments':
At revision 89001.

P:\Python\cpython\Doc>
"""

Thus, a usable workaround is to enable the eol extension in hgrc and make your checkout that way.  However, this doesn't help anyone who is trying to build from a tarball; Doc/make.bat will have LF line endings and at least 'update' will not work.

Perhaps a less extensive change would be to add "**.bat  = BIN", and then convert Doc/make.bat to CRLF explicitly?