bug#72300: project.el: detect newly created project contained within another

[Federico Tedin via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

In Emacs master e56e4b345a2, `emacs -q`:

I'm having problems trying to make project.el detect a new project that
is contained in the directory of another project.

I have a directory called 'scratch' which contains a '.git' directory, a
file 'test.py' and a directory 'foo'. The 'foo' directory contains a
file called 'foo.py'.


~/scratch/
    .git/
    main.py
    foo/
        foo.py


If I open 'main.py', `(project-current)' evals to the expected: `(vc Git
"~/scratch")'.

If I open 'foo.py', `(project-current)' also evals to `(vc Git
"~/scratch")', which is expected.

However if now I cd into 'foo/' and run `git init`, then I would expect
project.el to now consider 'foo.py' to be in another project - `(vc Git
"~/scratch/foo")'. However, if I evaluate `(project-current)' when
visiting 'foo.py', I still get `(vc Git "~/scratch")'.

If I kill the buffer visiting 'foo.py' and open the file again, I get
the same result.

Interestingly, if I run 'M-x project-remember-projects-under' with
'~/scratch/foo' as path, it does inform me that the new project has been
found. However visiting 'foo.py` still results in `(vc Git "~/scratch")'
as the current project.

If I restart Emacs then the problem is solved; 'foo.py' is correctly
filed under project `(vc Git "~/scratch/foo")'.

The fact that this works correctly after restarting makes me think
that there must be some runtime state set up that is preventing the
desired behaviour to happen.