normalize_path: Squash leading slashes even without allow_leading

[faho]

This currently changes builtin realpath with the "-s" option:

builtin realpath -s ///tmp

previously would print "///tmp", now it prints "/tmp".

The only thing "allow_leading_double_slashes" does is allow two
slashes.

This is important for path match, to be introduced in #8265.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

Okay, the only use of this is in builtin realpath, all other callsites allow leading double slashes.

So I feel comfortable merging this.