Don't update `.pot` file if only date changed

Since we're updating the transulations with a pre-commit hook, we
are constantly regenerating the file. This means potentially there
will be a change committed to the `.pot` file with every commit.

When no strings are changed, the POT-Creation-Date line will be
updated. With this change, we are checking for a 1 insertion, 1
deletion change in that file. If that is the case, don't stage the
`.pot` file.