Allow commits to linked pakages.

If the package is linked to a different project, osc will ask
permision to branch the package in the current project before
commiting.

Show a message in osc meta pkg -e in the same case.
- fixed core.edit_meta

The change introduced in commit a20c6f5d99287f529d2a290b2953cd41b027efff
broke the creation of new packages.
- Package.commit: fixed spelling error and added FIXME comment