The git plugin 2.4.0 and git client plugin 1.18.0 were released 18 Jul 2015 with this enhancement.

The git plugin and git client plugin are being tested in hopes of releasing new versions before the end of June. If you're willing to assist with the testing, please download and install a pre-release build of the git client plugin and the git plugin. Problems detected in the pre-release should be e-mailed to Mark Waite and Nicolas De Loof.

I wrote some test ideas if you would like suggestions of areas that need testing. The git plugin supports many different use cases and its automated tests only evaluate a very few of those use cases.

Rebased into git plugin 2.3.5 development snapshot.

Code changed in jenkins
User: Martin d'Anjou
Path:
src/main/java/hudson/plugins/git/GitSCM.java
src/main/java/hudson/plugins/git/UserMergeOptions.java
src/main/java/hudson/plugins/git/extensions/impl/PreBuildMerge.java
src/main/java/hudson/plugins/git/opt/PreBuildMergeOptions.java
src/main/resources/hudson/plugins/git/UserMergeOptions/config.jelly
src/main/resources/hudson/plugins/git/UserMergeOptions/help-fastForwardMode.html
src/test/java/hudson/plugins/git/AbstractGitTestCase.java
src/test/java/hudson/plugins/git/GitPublisherTest.java
src/test/java/hudson/plugins/git/GitSCMTest.java
src/test/java/hudson/plugins/git/opt/PreBuildMergeOptionsTest.java
http://jenkins-ci.org/commit/git-plugin/3d67ef2019cb8d1bb639575744a94ccea949a55c
Log:
JENKINS-12402 Support fast-forward mode configuration

Released in git-client-plugin 1.16.1

Code changed in jenkins
User: Mark Waite
Path:
.gitignore
src/main/java/org/jenkinsci/plugins/gitclient/CliGitAPIImpl.java
src/main/java/org/jenkinsci/plugins/gitclient/JGitAPIImpl.java
src/main/java/org/jenkinsci/plugins/gitclient/MergeCommand.java
src/test/java/org/jenkinsci/plugins/gitclient/GitAPITestCase.java
http://jenkins-ci.org/commit/git-client-plugin/a18a90e57d60c342fe1cfba2e5be24bd515e03aa
Log:
Merge branch 'feature/JENKINS-12402-add-option-for-no-ff-when-merging' of https://github.com/martinda/git-client-plugin into master-martinda-feature/JENKINS-12402-add-option-for-no-ff-when-merging

Compare: https://github.com/jenkinsci/git-client-plugin/compare/d1cded9f601d...a18a90e57d60

This would be very helpful. We use a hook to prevent modifications to our master branch unless the HEAD commit is by Jenkins. So long as there is one commit on master which is not on "next", that commit is automatically generated. If users accidentally merge master back into next, the lack of "--no-ff" means that there will be no easy way to tell if a modification of "master" is from Jenkins.