Fix malformed patch format by ensuring correct newline handling

[snomos]

This PR fixes the "malformed patch at line X" error that was occurring when running gut template apply commands.

Problem

The patch generation was creating malformed patches with trailing empty lines, causing the patch command to fail with errors like:

patch: **** malformed patch at line 29:  

Root Cause

The issue was in the to_content() functions in patch_file.rs:

1. Git diff content already contained newlines
2. When formatting patch lines with prefixes (+, -, space), we were getting double newlines or malformed endings
3. The final patch file was ending with empty lines instead of a single newline

Solution

This fix ensures proper patch formatting by:

• Cleaning content: Remove existing newlines before adding prefixes to avoid double newlines
• Consistent line endings: Each patch line gets exactly one newline after the prefix
• Proper file termination: Final patch ends with exactly one newline (no trailing empty lines)
• Header preservation: Hunk and Info lines preserve their existing newlines correctly

Testing

• Code compiles without errors
• Addresses the specific malformed patch errors reported in template apply operations

The fix maintains compatibility with the existing patch format while ensuring proper newline handling throughout the patch generation process.

[dylanhand]

Looks fine to me. I simplified the code slightly.