patch 9.2.0446: runtime(netrw): off-by-one bug in s:NetrwUnMarkFile()

patch 9.2.0446: runtime(netrw): off-by-one bug in s:NetrwUnMarkFile()

Problem:  off-by-one bug in s:NetrwUnMarkFile()
Solution: Correctly loop through all buffers to unlet all variables
          (J. Paulo Seibt)

When the function loops through buffers to clear s:netrwmarkfilelist_#
and s:netrwmarkfilemtch_#, it skips the last one at bufnr('$'), messing
up mark highlights and causing other functions that operate on those
arrays (like delete or rename) to target stale marked files.

The bufnr() help page says that bufnr("$") returns the highest buffer
number of existing buffers, so while ibuf < bufnr("$") does not clear
the last buffer-local arrays.

To reproduce:

Just opening a fresh Vim and running :Ex opens a netrw buffer at the
highest number. Then, typing mu after marking some files triggers the
mark highlight bug, and finally typing D would act like calling the
delete function against the previous marked files, as the buffer-local
arrays where not touched by s:NetrwUnMarkFile.

closes: #20129

Signed-off-by: J. Paulo Seibt <jpseibt@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>