update-git-snapshot: use a release version number

I initially used '-S' since I expected to do merges and no actual
releaes. It turns out the most tests are done on the librem5 branh and
once succesfull the deb is rolled out. So let gbp-dch figure out the
release version number (based on the git tag) and we only append the
+librem5.1 at the end (which could be done with the script later on
too).

This also gives the right version number when only changing parts of the
Debian packaging.
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