Support uploading to a specific branch

Currently all files are uploaded to the default branch, as that is what the GitHub API uses by default. I want to pass in a parameter indicating the branch name to use for uploading the file.

As a requirement I'd say that the branch already has to exist. Creating it is not necessary for me.

Changes needed

Add an optional parameter to the action to specify the branch name
Add the extra parameter to the PUT call in upload.js, see documentation

github-upload-action/upload.js

Lines 49 to 54 in 20142a0

    
             data: { 
        
               message: commitMessage, 
        
               sha, 
        
               content: base64Content 
        
             } 
        
           }).then(({ data }) => {

I'd be happy to help working on this :-)

I'd be thankful if you can make a PR for this.
Additional, There may be several points to note:

What will the GitHub API do if the branch is not existed?
Which structure should we use for the extra parameter to the PUT call?

I'd say create the branch if it doesn't exist yet. Currently the upload just fails with a non-descriptive error. Will see if I can add that as well, maybe tomorrow.

See the progress here #5

	data: {
	message: commitMessage,
	sha,
	content: base64Content
	}
	}).then(({ data }) => {