fix ReplacePlaceholderString

[Ryooooooga]

• PR Description

1. echo a >'a{{.filename}}b'
2. launch lazygit
3. press e on file a{{.filename}}b
4. aa{{.filename}}bb is opened in the editor

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go run scripts/cheatsheet/main.go generate)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[jesseduffield]

makes sense: so the original issue was that we were re-processing the string after the placeholder had been resolved. And in the new approach, we're not doing that.

oldnews was a confusing variable name to me at first but given the NewReplacer function itself expects an oldnew repeated arg it makes sense.