Better defaulting of username when not specified in https remotes

When using https remote URLs, SourceTree offers the ability to remember the password in the keychain.

However the keychain requires that you know the username before you can retrieve the password, and not all URLs include the username (especially Kiln).

In order to get around this, SourceTree stores a default username for each domain which it uses to try to look up the entry in the keychain if it's not in the URL explicitly. However if this can be ambiguous, e.g. multiple user names on a domain or a mistake in the log in window when first used, this can fail and keep prompting the user for credentials.

The most reliable way to solve this is to add the username to the URL, as in https://username@host.com/path/to/repo. However this is not necessarily intuitive to a user, and is not the default in Kiln Hosted Projects.

To solve this, it would be better to make the defaulting more effective - taking into account more of the URL in order to clarify ambiguous cases, and overwriting the setting with the last successful use in all cases so it doesn't get out of date.