A new module is added to jj_lib which exposes a function
get_annotation_for_file. This annotates the given file line by line with
commit information according to the commit that made the most recent
change to the line.
Similarly, a new command is added to the CLI called `jj file annotate` which
accepts a file path. It then prints out line by line the commit
information for the line and the line itself. Specific commit
information can be configured via the templates.annotate_commit_summary
config variable
