Set up default mappings with SourceControl.Git.Settings

[isc-pbarton]

Say I am configuring an instance programmatically with a setup script using the SourceControl.Git.Settings API:

set settings = ##class(SourceControl.Git.Settings).%New()
do settings.%Save()

Most settings values are set to a default value, but the mappings are not set to anything. Instead I have to set each mapping individually on the Mappings property.
To make this more convenient we could check if there are any existing mappings configured. If not, set Mappings to the default values.

[isc-tleavitt]

Relates to #225 re: backward-compatibility "if no mappings are configured" - might make sense to tackle simultaneously.

[isc-rsaptars]

Fixed