datalad siblings enable fails in git-cloned dataset without git-annex branch

[adswa]

If one clones a datalad dataset via a plain git clone, and one's local repo thus only has the master branch, running datalad siblings enable can crash and bubbles up the git error - the underlying reason is that the local repo doesn't yet see the git-annex branch.

(handbook-upgrade) adina@muninn in /tmp/argw on git:master
❱ datalad siblings enable -s osf-storage                                    1 !
CommandError: 'git -c diff.ignoreSubmodules=none -c core.quotepath=false cat-file blob git-annex:remote.log' failed with exitcode 128
fatal: invalid object name 'git-annex'.
(handbook-upgrade) adina@muninn in /tmp/argw on git:master
❱ git branch                                                              128 !
* master

[adswa]

this is a variant of #6569. Git's error message changed from "fatal: path 'remote.log' does not exist in 'git-annex'" to "fatal: invalid object name 'git-annex'".