It’s telling you that you have a branch named ‘branch_name’ and also a file or a directory named ‘branch_name’.
If you want the command to treat ‘branch_name’ as a branch use
git log --oneline branch_name --
if you want it to treat ‘branch_name’ as a file use
git log --oneline -- branch_name
If in case any one faced when trying the following and got above error, here is the fix
Problem:-
#In master branch
git checkout -b feature-a
#changed to feature-a branch
vi a.txt
git commit -m "adding a file" a.txt
git push
git diff master
fatal: ambiguous argument 'master': both revision and filename
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Solution:-
git diff origin/master