README doesn't explain how to not add files to your repo about gistup

[mbostock]

Three ways to do this are (indirectly) described in the README:

1. Use gistup -i to confirm each untracked file interactively.

2. Specify the files you wish to add explicitly as gistup -- file1 file2, rather than the default behavior of adding untracked files. If you specify a file that you’ve previously added and committed to the git repository, the initial gistup commit should be empty.

3. Use gistup -x '*' to ignore untracked files in the current directory. (Gistup already ignores untracked files in subdirectories because Gists aren’t supposed to have subdirectories.)