bitbake: fetch2/git: Prevent git fetcher from fetching gitlab repository metadata

The bitbake git fetcher currently fetches 'refs/*:refs/*', i.e. every
single object in the remote repository. This works poorly with gitlab
and github, which use the remote git repository to track its metadata
like merge requests, CI pipelines and such.

Specifically, gitlab generates refs/merge-requests/*, refs/pipelines/*
and refs/keep-around/* and they all contain massive amount of data that
are useless for the bitbake build purposes. The amount of useless data
can in fact be so massive (e.g. with FDO mesa.git repository) that some
proxies may outright terminate the 'git fetch' connection, and make it
appear as if bitbake got stuck on 'git fetch' with no output.

To avoid fetching all these useless metadata, tweak the git fetcher such
that it only fetches refs/heads/* and refs/tags/* . Avoid using negative
refspecs as those are only available in new git versions.

Per feedback on the ML, Gerrit may push commits outsides of branches or
tags during CI runs, which currently works with the 'nobranch=1' fetcher
parameter. To retain this functionality, keep fetching everything in case
the 'nobranch=1' is present. This still avoids fetching massive amount of
data in the common case, since 'nobranch=1' is rare. Update 'nobranch'
documentation.

Reviewed-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
(Bitbake rev: d32e5b0ec2ab85ffad7e56ac5b3160860b732556)

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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