Bug 492852

SUMMARY
```
19ca36b7fa135e5db107d63fe22197519be30441 is the first bad commit
commit 19ca36b7fa135e5db107d63fe22197519be30441
Author: Michael Reeves <reeves.87@gmail.com>
Date:   Fri Aug 23 18:51:29 2024 -0400

    Only set BOM for utf-16 or utf-32

 src/EncodedDataStream.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
```

@mreeves 

I bisected to this commit and it's bad: This commit corrupts the output file encoding on (3-way) merges. I consider that after this commit the trust worthiness of the output of kdiff3 is pretty much zero: If the kdiff3 manages to fully auto-merge, the user won't see the silent corruption at worst, and at best this is a source of numerous other bugs down the line.

I started the bisect process after testing commit 2209f1ebe56ebe4f3e5be9c447337a618ab5707e which otherwise worked, but courrupts the output file.

I would suggest reverting this commit, as the parent commit c53d6cda1efedbe8b3deb12f8efd077f03cd32d3 has no such issues.

STEPS TO REPRODUCE
1.  (Three-way) merge a file
2.  Save the file at any point in kdiff3

OBSERVED RESULT

Editors show BOM as enabled, even if original (unmerged) encoding was UTF-8 with no BOM.

The entire file is silently modified - it looks mostly ""ok"" at glance, but the text encoding is all wonky, and comparing before/after with 'diff' shows the entire file got rewritten. E.g. Kate, KDevelop, Geany cannot highlight/parse a such source file/lines correctly.

EXPECTED RESULT

Unmodified lines are not touched, unsaved file(s) are not modified. diff shows expected difference between before/after resolving conflicts.