Source-git packages whose .gear/rules builds the source tarball from the
version tag (e.g. 'tar: v@version@:.') do not track the upstream source in
the packaging branch -- it is regenerated at build time. Merging the upstream
tag with a regular merge then either fails with modify/delete conflicts (the
branch "deleted" the source files) or leaks new upstream files into the tree.

Use 'git merge -s ours' for these packages so the tag becomes an ancestor
(required for merge-base, changelog and gear-update-tag) without touching the
packaging tree. Packages that track source in the branch (vendored /
predownloaded, rule like 'tar: .gear/...') keep getting a regular real merge.

The discriminator is the @version@ tar rule, not the .gear/tags directory: a
previous attempt (d223e6ca) keyed on .gear/tags and was reverted (bd83b595)
because it matched vendored packages too and discarded their merged sources.

Add is_tar_from_version_tag_rule() to the gear module and cover it with bats
regression tests pinning the source-git vs. vendored distinction.