Revert "ExternalProject: Strip out "origin/" from git tag"

This reverts commit 40bf640cec545d4cbc4728b6fae78525ebd66368.
ExternalProject: Allow tags in the form <remote>/<tag>

If tag is in the form <remote>/<tag> (i.e. origin/master) they are
split and used for the pull.
ExternalProject: rebase instead of pull --rebase

Since fetch was executed a few lines before, it is not necessary to
contact the remote repository again.
Merge topic 'ExternalProject_GitUpdate' into next

2cc64ce6 ExternalProject: rebase instead of pull --rebase
c5ebe575 ExternalProject: Allow tags in the form <remote>/<tag>
bef58048 Revert "ExternalProject: Strip out "origin/" from git tag"