The previous override_dh_auto_build used 'git clone --branch <tag>'
which only accepts branch names. Feeding it 1.0.1-rc02 (a tag,
not yet pushed to origin) failed with 'La branche distante
1.0.1-rc02 n'a pas ete trouvee' and aborted the package build.
Switch to a 'git init + remote add + fetch --depth=1 origin <ref>
+ checkout FETCH_HEAD' sequence, which works for both branches
and tags, including local-only tags fetched via file:// (the
typical RC-staging workflow: 'git tag' on the local checkout,
then 'make POSTIT_GIT_URL=file:///... POSTIT_GIT_TAG=<tag>'
before pushing).
The fetch error now carries a diagnostic that points the operator
at the file:// override and at the missing 'git push'.
Adds tests/test-fetch-upstream.sh with four cases against the
real pazof/yavsc remote and a synthetic local repo: tag on
GitHub, branch on GitHub, .git reuse, and tag via file://.
Wired into 'make test'.
