At my organization we often prepend our commit messages with the key or keys of the JIRA issue(s) the commit addresses. Something like this if the commit addresses one issue...

[MYJIRAPROJECT-123]: My commit message.

Or this if the commit addresses multiple issues...

[MYJIRAPROJECT-123, MYJIRAPROJECT-456, SOMEOTHERJIRAPROJECT-789]: My other commit message.

We should probably be entering this issue data in the commit message in a more easily parsed format, like a JSON array...

    [
        "MYJIRAPROJECT-123",
        "MYJIRAPROJECT-456",
        "SOMEOTHERJIRAPROJECT-789"
    ]

It would be nice if I could give SourceTree the address of my JIRA server and then SourceTree could provide a drop down list of available issues when I make my commit. SourceTree would then automatically prepend my commit message with a JSON string like the one above.

There are a lot of things that could be done with this information, from SourceTree and from JIRA.

SourceTree could provide a hyperlink to the JIRA issues that were linked to by the commit. When the commit was pushed to master a JIRA hook could be implemented that would automatically create a hyperlink in any JIRA issues mentioned in the commit's JSON string. When clicked the hyperlink would cause SourceTree to open and navigate directly to the commit in question.