Removed git branches that weren't processed on clone fetch might not get marked as removed later on

Usually branches are marked as removed during fetching changes to the local git clone.

If the branches aren't marked as deleted during fetch (for example because the scanning was interrupted after fetch, but before saving changes), these branches will then produce a warning during scan:

WARN fisheye DvcsScanner-updateBranchHead - Unexpected branch removal: oldBranch=REMOVED-BRANCH@1234, newBranch=null. Ignoring.

Such branches won't ever get cleaned up, will produce a warning on each slurp, and might still appear in branch selectors. This shouldn't affect the results of the scan, and new changesets should appear normally.