XCode source control: view changes for one file
What it sounds like you want is the "Blame View", if we were on the terminal you could write git blame <file-name> to see the commit/time/and user for every single line of a particular file. In Xcode you can accomplish the same thing by activating the blame view. I've posted a picture to show you where the Blame View button resides. From there you should be all set.
For xcode 11.0, If you select a file, changes for that file is available on right panel.
The commit history is in the Version Editor.
Select the file for which you want to see the commit history. Then go to View > Version Editor > Show Version Editor (or just select the Comparison view from the top-right toolbar icon - same place as the Blame view).
Clicking on the commit label below the editor will reveal the history including dates.