bug#48324: 27.2; hexl-mode duplicates the UTF-8 BOM

[R. Diez]

I think that hexl-mode has problems with the UTF-8 BOM byte sequence at the 
beginning of a text file. The steps to reproduce this issue are:

Create a text file with a single line with 3 characters: 123

Do a (set-buffer-file-coding-system 'utf-8-with-signature-dos) and save the 
file.

The file should now have the following contents (8 bytes):

ef bb bf 31 32 33 0d 0a

That is the UTF-8 BOM (ef bb bf), the ASCII digits 1, 2 and 3, and end-of-line 
sequence (CR LF).

Now change to hexl-mode, place the cursor at the '1' character (31 in hex), call hexl-insert-hex-char, and enter 00 in order to replace the '1' with a 
binary zero (NUL character).

The result is puzzling. Instead of replacing the '1' (31) with NUL (00), the UTF-8 BOM is duplicated, the characters '1' and '2' and '3' have been 
overwritten with the new copy of BOM, character CR has been replaced with NUL, and character LF is intact:

ef bb bf ef bb bf 00 0a

If you save, close and reload the file, it gains one byte, but that is probably 
not important, just a consequence of having lost the CR character:

ef bb bf ef bb bf 00 0d 0a