CheckJira: improve githooks.checkjira.project documentation
Downcase first letter of all error messages

Most of them were already lowercase. I just downcase the ones that were
in uppercase.
CheckStructure: cache result of get_structure
Uniformize commit citations in error messages

Error messages that want to refer to a particular commit now begin with
"commit SHA1 ...". If the SHA1 is unknown it's left out.
Uniformize error message endings

They don't end in a period and neither in a newline.
CheckJira: make githooks.checkjira.project strict

When this option was used, JIRAs for non-specified projects were simply
ignored, which isn't the best semantics. The user could be led to
believe that she was citing correct JIRAs when in reality they could not
even exist.

There are two other possible semantics for this option: it could either
reject any JIRA of a non-specified project or accept them if they
respected all other CheckJira checks.

It's not obvions which one is better but in the end I decided for the
more strict semantics, viz. to reject JIRAs of non-specified
projects. One argument for this is that JIRAs should respect ALL checks,
and the githooks.checkjira.project is such a check and should be
respected too. Another argument is that if the user wants to cite JIRAs
of other projects she is able to do so by citing them outside of the
place defined by the githooks.checkjira.matchlog option, since those
would not even be detected.

Note that this is a backwards-incompatible change. But it's really minor
and I don't think it'll have an impact. And if it has, it will be
for the better, I think.
Exempt a routine form perlcritic's complexity check

I'll get to refactoring it later.
v0.050

  [Changes]

  - CheckJira's githooks.checkjira.project option is now strict, i.e., when
    it's set specifying a set of JIRA projects only those project's issues
    may be refered to in the commit messages. Other project's issues are
    rejected. Previously, other project's issues were accepted without any
    other check.

  - Make error messages more uniform.