gitlab: aggressively avoid extra GIT data

gitlab: aggressively avoid extra GIT data

This avoids fetching blobs and tree references for branches we are not
going to worry about. Also skip tag references which are similarly not
useful and keep the default --prune. This keeps the .git data to
around 100M rather than the ~400M even a shallow clone takes.

So we can check the savings we also run a quick du while setting up
the build.

We also have to have special settings of GIT_FETCH_EXTRA_FLAGS for the
Windows build, the migration legacy test and the custom runners. In
the case of the custom runners we also move the free floating variable
to the runner template.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240312170011.1688444-1-alex.bennee@linaro.org>