bug#45200: Memory leaks: (garbage-collect) fails to reclaim memory

[Konstantin Kharlamov]

The problem is basically: (garbage-collect) refuses to collect some of the 
memory. It is more visible if you delay garbage collection, which is what I do 
for for performance-related reasons. My config has the following snippet:

    ;; only run garbage collection on idle
    (setq gc-cons-threshold most-positive-fixnum)
    (run-with-idle-timer 2 t (lambda () (garbage-collect)))

How much memory gets lost depends on configuration. For me right after start 
the difference is 40 MB: ≈60 MB is Emacs PSS size without above code, and ≈100 
MB it is when garbage-collection is delayed, *after* I run explicitly 
(garbage-collect). It is less visible without any other configs, nonetheless 
it's visible.

# Steps to reproduce:

1. Run `mkdir /tmp/.emacs.d`
2. Run emacs as `HOME=/tmp/ emacs`, and measure its PSS
3. Create a file /tmp/.emacs.d/early-init.el with content:

    ;; only run garbage collection on idle
    (setq gc-cons-threshold most-positive-fixnum)
    (run-with-idle-timer 2 t (lambda () (garbage-collect)))

4. Run emacs as `HOME=/tmp/ emacs`, evaluate (garbage-collect), then measure 
its PSS

## Expected

Size has no statistically-significant difference, because in both cases we 
garbage-collected memory.

## Actual

Size without calling explicit garbage-collect, from 3 runs, varied around 
41.2..41.7 MB.

Size afterwards, also 3 runs, varied around 45.4..45.5 MB.

This is 4 MB lost. While not much, but as I mentioned it grows as much as to 
40MB. The Emacs whose emacslient I'm using to write this email has size 218 MB, 
and now I wouldn't be surprised if much of that is actually a leaked memory.

# Version

Emacs-git 28.0.50, build from commit abd15e088e99