edit_file strips executable bit from existing scripts #179

When functions.edit_file modifies an existing executable file, it rewrites the file as mode 0644 instead of preserving mode 0755.

Observed in codedb on scripts/bench-ab.sh: each edit_file call changed the Git diff mode from 100755 to 100644; running chmod +x fixed it, but a later edit_file call stripped it again. The edit operation should preserve the existing file mode, especially executable bits.