bug#45748: 28.0.50; fit-frame-to-buffer ignores leading spaces

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Sat, 9 Jan 2021 11:55:06 -0600
> Cc: 45748@debbugs.gnu.org
> 
> It might actually be easier to read and understand if it was written
> without fetch_char_advance and just used inc_both, dec_both and
> FETCH_BYTE.

Yes, probably.

But before we do any changes here, we need a test suite.  Would you
mind adding the necessary tests to test/src/xdisp-tests.el?