blob_controller.rb

Currently when a user performs an update of a file through the UI  and there
has already been a change committed to the file the previous commits will be
overwritten without a check to see if the file has been changed.

This commit uses the last commit sha at the time the user starts editing the
file and compares it with the current sha of the file being edited to ensure
they are the same before committing the file. If the shas do not match we
throw an exception preventing the commit from the commit from occurring.

Fixes #5857