Some packages that depend on org (like elfeed) will load the built-in
org early in the rebuild/package install process, which causes org to
define org-release and org-git-version, sometimes overwriting our stubs
for it. Without our hack, org call 'git describe' in the org repo in an
attempt to determine the installed version, which won't work in a sparse
clone. To ensure future definitions never overwrite ours, we advise them
as well.
Also moves magit-version hack to its autoload file, for consistency with
org's hacks.
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