support/download/git: rename local refs to avoid confusing Git warning

support/download/git: rename local refs to avoid confusing Git warning

Running "git fetch origin ${cset}:${cset}" to create a local ref
${cset} from the remote ref ${cset} causes Git to issue a warning like
the below, when the version is a full commit hash:

===

warning: refname '49eb4ecb1ef9879ebc6789a1bdb536ab2b1d9871' is ambiguous.
Git normally never creates a ref that ends with 40 hex characters
because it will be ignored when you just specify 40-hex. These refs
may be created by mistake. For example,

  git switch -c $br $(git rev-parse ...)

where "$br" is somehow empty and a 40-hex ref is created. Please
examine these refs and maybe delete them. Turn this message off by
running "git config advice.objectNameWarning false"

===

This warning is very confusing for users, and is caused by the fact
that Git doesn't like our local ref name to look like a commit hash.

So, this commit proposes to fix the issue by having the local ref
named buildroot-${cset}, i.e
buildroot-${version-specified-by-the-package}.

The generated tarballs are exactly identical, nothing changes, it is
really just internally the local ref we are using to checkout the
correct version that is different. And it avoids the confusing
warning.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>