The git-branch recipe says: »Note that the next release will probably
allow repo.listall_branches().« Concluding from the README,
Repository.listall_branches() was included in some release prior to
0.20.0, so at least that statement is obsolete.
However, since pygit2.org brings up fairly accurate results for a search
on »list all branches«, I figured that the whole recipe isn't needed
anymore. Therefore delete it.
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