Use existing hg/git credentials stored in the keyring

[philipstarkey]

We currently store credentials in the keyring, but some credentials for certain accounts/repositories are already there (if you use git/mercurial/tortoisehg on your PC). We should look to see if we can use those rather than asking the user to enter them again.

This is particularly useful given that the hg/git clone stage doesn't use the credentials entered into this tool and instead uses whatever is in the keyring for those programs (or prompts the user to enter them).