Note the dependency on PCRE support in Git implied by the use of grep -P in the git hooks

I'm working through gotchas I hit, in the hope that others can avoid them. In this case, building git from source will not get the -P option available in grep, that's used by shellcheck_all. An alternative approach would be to change the regular expression in shellcheck_all such that it does not need Perl compatible regular expressions. I don't know if that is feasible.