Improvements to p4-annotate:
1. Show error messages if you try to annotate zero (or two or more) files.
2. Jump to the current line in the annotation buffer if that makes sense in context. (This means there is no longer any need for a separate p4-annotate-line command.)
3. Go to the correct line in the annotation buffer.
4. Don't rerun the annotation if there's already a buffer displaying it.
Fixes #119.