Allow existing git amend alias to override the spr version

[alexeyr-ci]

Not sure if it's possible in the first place (I expect commands always have priority over aliases with the same name) or too much effort even if it is. But I have a git amend alias I use constantly and it's in my muscle memory. So the ideal for me would be:

1. git spr amend as an alternative to the current git amend. This way all the functionality of spr is available under git spr.
2. If amend is already defined as an alias, running git amend should run the alias. If it isn't, run the spr version.