Add settings to use git branch for current save

[typeholes]

Automatically switching saves in the persistent storage to the current head label would be an easy way to have traces and trace diagnostics match the current code

It is possible to get the label from the built-in git extension, but a quick look didn't find a change event

[typeholes]

It may be better to let the user specify a shell command to generate a save name. Not everyone uses git, and even git users may want something other than the head label

[typeholes]

Also need to find a good fallback for a default when a git repo is not found