build: show branch on skip in search_package()
When dealing with EXTRABRANCHES the output can be confusing when
a branch was not updated but we don't know which one.
build: add REPO_STRIP=<tag> helper to git_fetch
(this is a re-commit since git doesn't like pushes for non-bare
repository with the current branch being pushed also checked out)
build: add REPO_STRIP=<tag> helper to git_fetch
Note that the build system is NOT the reference git repository
and stripping a tag is non-fatal because it will be replicated
from the actual reference.