How can I list all commits that changed a specific file?

The --follow works for a particular file

git log --follow -- filename

Difference to other solutions given

Note that other solutions include git log path (without the --follow). That approach is handy if you want to track e.g. changes in a directory, but stumbles when files were renamed (thus use --follow filename).

git log path should do what you want. From the git log man:

[--] <path>…

Show only commits that affect any of the specified paths. To prevent confusion with 
options and branch names, paths may need to be prefixed with "-- " to separate them
from options or refnames.

I have been looking at this closely and all these answers don‘t seem to really show me all the commits across all the branches.

Here is what I have come up with by messing around with the gitk edit view options. This shows me all the commits for a file regardless of branch, local, reflog, and remote.

gitk --all --first-parent --remotes --reflog --author-date-order -- filename

It also works with git log:

git log --all --first-parent --remotes --reflog --author-date-order -- filename