Sets the URL hash to the address of the cursor when a user selects a
diff line (either by clicking the line number or using the 'c' hotkey).
If the cursor is on a line of the revision, the address is the line
number. If the cursor is on a line of the base patch, the address is the
letter 'b' followed by the line number. Otherwise the address is the
empty string.
Bug: Issue 4206
Change-Id: Ic85da197eb6264ea1111cc34e781dccbc24d4d40