Previously (and currently) using the TortoiseHg client for Windows, which is my past experience with Hg. In that client, you can go through your list of files before commiting and press the Space key to select which ones to commit.
In SourceTree, I don't seem to find the keyboard shortcut for this. It is just me or is this function not supported?
(I tend to prefer by working with keyboard rather than mouse as much as possible.)
On the Mac the shortcuts are:
Command-Shift-+ (plus): stage selected files for commit (use the '+' key above '=', not the numeric keypad '+')
Command-Shift-- (minus): un-stage selected staged files
These work in the log view and the file status (commit) view.
There are also shortcuts for selecting files:
Command-A: select all files
Up arrow, Down arrow, space bar: move to and select files
Unfortunately the keys for selecting files only work if the keyboard focus is on the files pane. I can't figure out a way to put focus on that pane from the keyboard. So close! Urgh.