[DOC] merge-file: document that merge.conflictStyle requires a repository

Hi,

We noticed that `git merge-file` only respects the `merge.conflictStyle` configuration when run inside a repository. Outside a repository, the setting is ignored and only the `--diff3`/`--zdiff3` flags work.

Looking at the history, this appears to be intentional since b541248467 ("merge.conflictstyle: choose between merge and diff3 -m styles", 2008), which explicitly gates config reading on being inside a repository.

This behavior surprised me, and I couldn't find it documented anywhere. Would a small documentation patch to git-merge-file.txt be welcome, noting that the config is only read when inside a repository?

Something like adding to the --diff3 option description:

  When run inside a repository, the `merge.conflictStyle`
  configuration variable can be used as a default for this option.

Thanks,

Yannik Tausch