glab mr create fill option hides some functionality

This issue is more of a documentation issue than anything else. I will lay out the problem I encountered to give a little context as to what happened.

In one of our pipelines we have some changes being made that need to be committed to a separate branch and then have a MR created with those changes. The way in which the branch is created is not to create the branch on the running job in the pipeline but just to push directly to a remote from the current branch: git push origin "HEAD:refs/heads/new-branch". This way we don't have to worry about creating a branch from the gitlab runner job.

The very next step is to create the MR with glab: glab mr create -b old-branch -s new-branch --remove-source-branch -f. When that command is run we get an error:

Failed to create merge request. Created recovery file: /path/to/mr.json
Run the command again with the '--recover' option to retry
fatal: ambiguous argument 'old-branch...new-branch': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
git: exit status 128

After digging thru the glab source code I noticed with the -f/--fill option it will also set the --push option behind the scenes. This causes glab to try and push a branch it does not currently have locally and fail. The workaround for this is to not use the --fill option and just supply the --title and --description options.

It is not clear this option is being set by reading thru documentation. At the very least documentation should reflect this and let the user know the --push option is also set when setting the --fill option. Another option would be to allow setting --push=false to unset this but this should still be documented.