The repo::template test suite makes use of quite a few local variables
that could be consolidated. Do so to make the code easier to read.

All tests in repo::template have a common pattern of first setting up
templates, then settung up the repository that makes use of those
templates via several init options. Refactor this pattern into two
functions `setup_templates` and `setup_repo` that handle most of that
logic to make it easier to spot what a test actually wants to check.

Furthermore, this also refactors how we clean up after the tests.
Previously, it was a combination of manually calling
`cl_fixture_cleanup` and `cl_set_cleanup`, which really is kind of hard
to read. This commit refactors this to instead provide the cleaning
parameters in the setup functions. All cleanups are then performed in
the suite's cleanup function.

Update `git_repository_init_ext` to use our typical style of error
handling. The function had multiple statements which didn't `goto out`
immediately but instead deferred it to later calls combined with `if`
statements.

When using templates to initialize a git repository, then git-init(1)
will copy over all contents of the template directory. These will be
preferred over the default ones created by git-init(1). While we mostly
do the same, there is the exception of "HEAD". While we do copy over the
template's HEAD file, afterwards we'll immediately re-initialize its
contents with either the default "ref: refs/origin/master" or the init
option's `initial_head` field.

Let's fix the inconsistency with upstream git-init(1) by not overwriting
the template HEAD, but only if the user hasn't set `opts.initial_head`.
If the `initial_head` field has been supplied, we should use that
indifferent from whether the template contained a HEAD file or not. Add
tests to verify we correctly use the template directory's HEAD file and
that `initial_head` overrides the template.