Git flow finish feature issue on Mountain Lion

Raised at https://answers.atlassian.com/questions/83511/sourcetree-cannot-find-git-flow-osx-mountain-lion#comment-96195

Using OSX 10.8.2, SourceTree 1.5.4, embedded git and git-flow:

The git-flow operation mentioned above (finish feature with rebase), now produces this error:

Will try to rebase 'testfeature'...
git: 'rebase' is not a git command. See 'git --help'.
Finish was aborted due to conflicts during rebase.
Please finish the rebase manually now.
When finished, re-run:
git flow feature finish 'testfeature' 'develop'
I used the test case from the associated bug report:

https://jira.atlassian.com/browse/SRCTREE-1183

Steps:

unpack the test repo and import into SourceTree
check out feature/testfeature
run Git Flow -> Finish Current for Feature: testfeature.
check "Rebase on development" and click "OK"
Of course, I see the same problem in my own repos as well.

Perhaps the cause (additions to PATH required) is the same? External Git + external git-flow works fine, e.g., git flow feature finish -r testfeature.