feat: strip prefix

[JP-Ellis]

If fixups are allowed, then the remainder of the commit message ought to either be either a previous commit's subject line, or a hash:

If a commit message starts with "squash! ", "fixup! " or "amend! ", the remainder of the subject line is taken as a commit specifier, which matches a previous commit if it matches the subject line or the hash of that commit.

So we can either:

1. Assume that previous commit's subject line was previously validated and do nothing further; or,
2. Validate the remainder of the commit's subject line as is.

This specific implementation opts for the latter.

Resolves #392

[coveralls]

Pull Request Test Coverage Report for Build 12395677344

Details

• 0 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 4.948%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/committed/src/checks.rs	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
crates/committed/src/checks.rs	1	1.79%

Totals
Change from base Build 12125583972:	-0.05%
Covered Lines:	24
Relevant Lines:	485

---

💛 - Coveralls

[JP-Ellis]

I've rebased this PR on top of master in case you're still interested in merging this.