I wanted to use the "automatic merging" functionality as described here: "Using Git, Jenkins and pre-build branch merging" ( https://wiki.jenkins-ci.org/display/JENKINS/Git+Plugin#GitPlugin-AdvancedFeatures ). I configured my job a described with the integration branch (Branch to merge to) configured to "developAutoMerge" and leaved the 'branch' field in the Git SCM blank.

When I "Build now" I get following exception:

Fetching changes from the remote Git repository
Fetching upstream changes from git@REPOSITORY.git
Seen branch in repository origin/develop
Seen branch in repository origin/developAutoMerge
Seen branch in repository origin/feature/blabla
[...]
Seen branch in repository origin/master
Seen branch in repository origin/release/2.6.1
Seen branch in repository origin/release/2.6.2
Seen 13 remote branches
Multiple candidate revisions
Scheduling another build to catch up with blaServerBuild_developAutoMerge
Merging Revision 6474f8ef91822e58edc55aad707d2725ff5a8431 (origin/feature/blabla) onto /developAutoMerge using resolve strategy
FATAL: Command "git rev-parse /developAutoMerge^

{commit}" returned status code 128:
stdout: /developAutoMerge^{commit}

stderr: fatal: ambiguous argument '/developAutoMerge^

{commit}': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions

hudson.plugins.git.GitException: Command "git rev-parse /developAutoMerge^{commit}

" returned status code 128:
stdout: /developAutoMerge^

{commit}

stderr: fatal: ambiguous argument '/developAutoMerge^{commit}

': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions

at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:1148)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:1125)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:1121)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommand(CliGitAPIImpl.java:937)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommand(CliGitAPIImpl.java:947)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.revParse(CliGitAPIImpl.java:401)
at hudson.plugins.git.GitAPI.revParse(GitAPI.java:257)
at hudson.plugins.git.extensions.impl.PreBuildMerge.decorateRevisionToBuild(PreBuildMerge.java:62)
at hudson.plugins.git.GitSCM.determineRevisionToBuild(GitSCM.java:795)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:862)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1415)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:652)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:561)
at hudson.model.Run.execute(Run.java:1678)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:231)

I found a similar problem by googling: https://groups.google.com/forum/#!topic/jenkinsci-dev/ek4hYR-z08k
The proposed solution there was downgrading the Jenkins Git plugin from 2.0 to 1.4.0.