PR Description

The existing diff parser incorrectly treated subsequent lines beginning with --- as filename headers while processing hunks. This caused corruption when dashed lines appeared within diffs themselves.

Restrict filename detection to only occur between hunks.

This repository https://github.com/abobov/lazygit-fixup-issue demonstrates the issue:

• clone repository
• run git reset --soft HEAD~
• try <c-f> Find base commit for fixup
• fatal: no such path ... error

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run 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)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc