Fixes #11934

A bigger issue than originally mentioned in #11934 is that currently TextEditor provided/used by GitExtensions actually corrupts files 😬

Proposed changes

• ICSharpCode.TextEditor has a helper which attempts to auto-detect the encoding, lets use it
• Depends on Default to an Encoding without BOM when BOM marker was not found ICSharpCode.TextEditor#42 because of this line FilePreamble = reader.CurrentEncoding.GetPreamble();
• For good measure, lets avoid creating MemoryStream when not necessary and we can write straight to FileStream

Screenshots

Before

Switching between UTF8 file without BOM and with BOM

After save using fileeditor (F4 from within commit dialog) ⚠️⚠️⚠️ notice corruption in a file without BOM

After

Both files even after saving them with fileeditor (F4 from within commit dialog) are recognized as UTF8

Test methodology

• Manually, checking files visually and with Hex editor

Test environment(s)

• Windows 11

Merge strategy

I agree that the maintainer squash merge this PR (if the commit message is clear).

✒️ I contribute this code under The Developer Certificate of Origin.