Hi,
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.)