View File History and Track Changes
You can view the history of changes in an individual file. To get a general picture, go to the History view. For a more detailed line-by-line analysis, check out the Annotate view.
The History view
The History view lets you see how a file has evolved since it was created and who has made changes to it.
To open the History view, navigate to the file and click 
on top of the file view.
The list of commits that have affected the file will be displayed:
To go back, use your browser Back button.
The Annotate view
The Annotate view displays line-level references to contributors and their commits that have recently affected specific lines of code in the file.
To open the Annotate view, navigate to the file and click the Annotate switch on top of the file view.
Each reference associated with a specific change contains a revision ID with a link, its author, and a timestamp.