I think I now understand my earlier problem. Selecting a file to edit by clicking on its name in the project drawer selects that file for any later Commit action. Once a file is selected, changing to another file by any means--clicking on its name in the file drawer, clicking on its tab, using Command-<digit>, using Option-Command-<arrow>, or selecting it from the Command-T list--will select the new file in the project drawer, and now Commits will be focused on that file.
I didn't realize any of this before; the subtlety that Allan corrected was way over my head.
So as I now understand it, the only way to keep Commit working on the entire folder (at present, my git repos have no subdirectories) is to select the folder in the project drawer and then move from file to file via the tabs or the various Command key shortcuts, as those are the only ways to change the frontmost file without changing the selection in the project drawer.
I'm with Thorsten; it isn't intuitive to me that using different ways of selecting a file for editing should have an effect on the Commit command.