Here's what I did:
1. install Jenkins
2. create a project (call it "Foo")
3. say, "whoops. forgot the Git plugin"
4. install the Git plugin through Jenkins
5. restart Jenkins
6. configure the repo in Foo
I kept getting the error:
Error trying to determine the git version: --version
It couldn't see the git binary. I set the git binary in the main Jenkins config to "git" and to "/usr/bin/git", with no change. I also confirmed that the jenkins user could run the git binary, which it could.
To fix it, I deleted the project and created a new one. After all that, apparently you have to install the Git plugin before you create the project.