Jenkins: Misc changes

[crawfxrd]

• Use GIT_COMMIT instead of stashing the commit
• Do not shadow built-in GIT_BRANCH variable with a parameter
• Update scmGit config based on docs

[crawfxrd]

It seems it's now the opposite problem that also affected the Freestyle Project: Polling SCM doesn't find any changes.

Turns out that's just a limitation of "Pipeline" projects. You need to use "Multibranch Pipeline" to support polling multiple branches.