Fix ExtMerge pattern for tags and octopus branches
Fix the ExtMerge pattern to handle more merge subject forms:
- Multiple branches from URL:
Merge branches '...' and '...' of ...://...
- Merging tags from URL:
Merge tag '...' of ...://...
Merge tags '...' and '...' of ...://...
- Merging URL into specific branch:
Merge ... of ...://... into ...
This allows treeplot to pick up many more external merges, especially in
recent releases, that would otherwise have been treated as internal
merges and often been accounted for in [No tree].
Signed-off-by: James Hogan <james.hogan@imgtec.com>