Rework head_whitesp job
This re-uses the same script from the Runner machine as is used for the
reworked cppcheck job. In an analogous way, this enables us to more easily
detect whitespace errors introduced in new work, without first requiring
the removal of any examples from pre-existing versions.
Due to the shared framework, this job has many of the same facilities as
the cppcheck job. You can get more information about its deductions by
setting VERBOSE to 1 when launching a pipeline manually. You can also
suppress diagnostics by commit, name, file or line; however a different
variable name is (re)used for this job: WHITESPACE_WHITELIST.
Rather than using a custom regular expression to detect whitespace errors,
this job now uses git diff-tree --check, so it picks up all the same
issues as git itself does.
Obsoletes !18 (closed).