It seems that in current version of SourceTree this functionality doesn't work again.

Original message:
My global .gitconfig file contains lines similar to this:

[commit]
template = /Users/kate.ebneter/.git_template.txt

When using command-line git, this causes the commit message to be populated with the contents of .git_template.txt. However, when committing via SourceTree, I get a blank commit message. It would be really nice if SourceTree honored this git setting.