Extend API to include ADD_OR_UPDATE operation

[adds68]

ostree/src/libostree/ostree-repo.h

Line 202 in 794d1d9

* OstreeRepoRemoteChange:

We have faced an issue in Buildstream, where we are adding a remote ostree repo to our build.

The issue in Buildstream occurs when a URL has been previously configured fora build, but the REMOTE_URL gets changed before the next build is run, the old REMOTE_URL is maintained, causing the new URL to be ignored as ADD_IF_NOT_EXISTS, fails to UPDATE the URL as of course it has already been set.

It would be nice to have a method, which checks to see if the current URL has been set, if it has, then it UPDATEs the URL, if it hasn't then it simply ADDs the current URL as the remote.

[cgwalters]

See #1166