Beware of this git(1) gotcha:

If you do

$ git log --oneline file.txt

you'll get your list of commits on file.txt. If you

$ git log --oneline -1 file.txt

you'll get just the first (most recent) one.

If you reverse your initial output

$ git log --oneline --reverse file.txt

you get things in the opposite order. But if you want the first one of those (the oldest modification of the file), you can't use -1 like

$ git log --oneline --reverse -1 file.txt

because as the man-page says, "Note that these [filtering criteria like `-1`] are applied **before** commit ordering and formatting options, such as --reverse" [emphasis mine]

So instead you need to use

$ git log --oneline --reverse file.txt | head -1