I had forgotten to update the comment when I wrote
357eb2484bf58ed202b072b94fff54d81fe0fb49.

The `sed -i` calls in the "change" and "remove" commands will overwrite
a symlinked database file with a real file, destroying the symlink.

To prevent this and allow the database file to be symlinked (to, say,
allow it to be tracked in a dotfiles repo), use a redirection technique
as described here:
https://www.cyberciti.biz/faq/howto-prevent-sed-i-from-destroying-symlinks-on-linux-unix/

Using this alternate method, we preserve symlinks, and even create a
backup file as before.

I was getting this error when running `qcd`'s change command on a
machine using Bash 4.4.12:

    $ sed -E "s/^shortcut       .+$/shortcut    ${p//\//\/}/" .config/qcd/database
    sed: 1: "s/^shortcut	.+$/shortcut ...": bad flag in substitute command: 'U'

The solution, as described here:
https://stackoverflow.com/questions/27787536/how-to-pass-a-variable-containing-slashes-to-sed/27788661#27788661
is to add an extra backslash to the parameter expansion. This appears to
work on both Bash 3 and 4 in my local manual tests.

Sometimes it's helpful to list the shortcuts you've added to help you
remember what they were. I had been doing `cat ~/.config/qcd/database`
for this, but it's nice to have it as part of the `qcd` command and also
to not have to type all that.