Installing the SourceTree Command Line Tools will provide you with the stree command. This will allow you to open the current directory in SourceTree.

You can also specify a particular path to a repo
stree ~/my-repo-in-another-folder
If installing command-line tools isn’t an option for whatever reason, you can also do the following:
open -a SourceTree path-to-file
and maybe set up an alias in .bashrc or .zshrc
alias sourcetree="open -a SourceTree"
For those who are using SourceTree 3
alias sourcetree="open -a SourceTree\ 3"