gh-116393: glob.translate match empty path parts with "**" when recursive=True

[ap--]

Closes #116393

Hello everyone,

this PR makes glob.translate match empty path parts (or consecutive path separators, i.e. "foo//bar") with "**" when recursive=True.

When I first implemented the change, I noticed that a pattern that starts with "**" would also match paths that start with "/", because they start with an 'empty part'.

So I added a negative lookahead to the beginning of the regex in case the pattern starts with "**", see: 1624308

This prevents glob.translate("**", recursive=True).match from matching "/foo", which is intuitive for me.

edit: "**" matching "/" is in line with pathlib

>>> import pathlib
>>> pathlib.Path("/").full_match("**")
True

Cheers,
Andreas

• Issue: glob.translate("a/**/c", recursive=True, include_hidden=True) does not match "a//c" #116393

[ap--]

I think that change introduced problems with pathlib. I'll work on a fix.

[ap--]

Okay, it looks like "**" matching "/foo" is intentional:

cpython/Lib/test/test_pathlib/test_pathlib_abc.py

Lines 497 to 498 in 5b2f21f

	self.assertTrue(P('/a/b/c.py').full_match('**'))
	self.assertTrue(P('/a/b/c.py').full_match('/**'))