Is your feature request related to a problem? Please describe.
Lets say we do not have tracked file. We we made dome changes to file, and we want to quickly see what lines were edited.

Describe the solution you'd like
Gitsign could higlight/mark somehow edited lines (comparing edited file to non edited version on HDD)

Describe alternatives you've considered
We could maybe run diff of current buffer to original version but signs could be nicer

Additional context
Cool implementation in kde Kate text editor:

Modified lines are marked yellow, on save they tun green, and then new additional lines are marked yellow again. We can repeat save and new changes turn gree.
I know this may be outside the scope of this plug but still it could be nice feature to have..