feat(cp): support custom commit message templates

[vjymisal0]

Summary

Add git cp --commit-template TEMPLATE. Every {} in the template is replaced with the default commit message, so text can go before or after it (e.g. --commit-template "#1252 {}" or --commit-template "{} [skip ci]"). The default is {}, so existing invocations keep their messages.

Verification

• bats tests/git-cp.bats: 4 passed (prefix, suffix, missing value, default invocation)
• Full bats ./tests suite: 88 passed
• shellcheck bin/git-cp: clean
• Man pages regenerated with Ronn-NG 0.9.0

Fixes #1252

[hyperupcall]

Thanks for working on this. I'm thinking that it may be useful to customize not just the prefix, but also the suffix as well. What do you think of passing --commit-template, with the default value being {}, which represents the message that is currently? There may be a better argument name or token which will be replaced with the substituted message.

[vjymisal0]

Good idea @hyperupcall, a template is more flexible than a prefix. I replaced --message-prefix with --commit-template, defaulting to {}. Every {} is replaced with the original commit message, so both --commit-template "#1252 {}" and --commit-template "{} [skip ci]" work. I kept {} as the token since it matches what you suggested, but happy to rename it if you have something else in mind.

Also regenerated git-cp.1/git-cp.html, which is what the lint job was complaining about. Added a suffix test; the full bats suite passes locally.

[hyperupcall]

Closing this since the contribution policies were not followed.