Improve the fidelity of `git reword

[epage]

• We should be editing a specific temp file
• There is specific behavior around this temp file
• We are ignoring GIT_EDITOR and the settings around it

See

• https://github.com/arxanas/git-branchless/issues/475#issuecomment-1381543394
• https://github.com/arxanas/git-branchless/blob/0c4ae541d13bccbf753b3a49bfdffb26418aa7db/git-branchless-lib/src/core/config.rs#L140