This was originally reported here and marked as fixed: 

https://issues.jenkins-ci.org/browse/JENKINS-18820?jql=text%20~%20%22default%20branch%22

However, the fix only seems to change the textbox: 

https://github.com/jenkinsci/git-plugin/commit/7758cde84d261403f6c8a31097544c8cc6792471

It doesn't fix the underlying behavior that the default shouldn't build all branches.

Looking at https://github.com/jenkinsci/git-plugin/blob/master/src/main/java/hudson/plugins/git/GitSCM.java it seems like this was tried on line 175:

this.branches = isEmpty(branches) ? newArrayList(new BranchSpec("*/master")) : branches;
 

However, the Constructor is called on line 155 with a list of one item:

public GitSCM(String repositoryUrl) {
this(
createRepoList(repositoryUrl, null),
Collections.singletonList(new BranchSpec("")),
false, Collections.<SubmoduleConfig>emptyList(),
null, null, Collections.<GitSCMExtension>emptyList());
}

Since this is a list of one item I think it passes the isEmpty() check and still defaults to building all branches instead of */master.

This recently caused a lot of pain, when someone accidentally did a build with parameters and didn't specify a branch. It then built every branch on origin overloading our build server.

I briefly tried to checkout the project to submit a pull request, but I couldn't get the project to build because of this error:

Gradle sync failed: Could not find org.jenkins-ci.tools:gradle-jpi-plugin:0.1-SNAPSHOT.
 Searched in the following locations:
 https://repo1.maven.org/maven2/org/jenkins-ci/tools/gradle-jpi-plugin/0.1-SNAPSHOT/maven-metadata.xml
 https://repo1.maven.org/maven2/org/jenkins-ci/tools/gradle-jpi-plugin/0.1-SNAPSHOT/gradle-jpi-plugin-0.1-SNAPSHOT.pom
 https://repo1.maven.org/maven2/org/jenkins-ci/tools/gradle-jpi-plugin/0.1-SNAPSHOT/gradle-jpi-plugin-0.1-SNAPSHOT.jar
 file:/Users/djbrick/.m2/repository/org/jenkins-ci/tools/gradle-jpi-plugin/0.1-SNAPSHOT/maven-metadata.xml
 file:/Users/djbrick/.m2/repository/org/jenkins-ci/tools/gradle-jpi-plugin/0.1-SNAPSHOT/gradle-jpi-plugin-0.1-SNAPSHOT.pom
 file:/Users/djbrick/.m2/repository/org/jenkins-ci/tools/gradle-jpi-plugin/0.1-SNAPSHOT/gradle-jpi-plugin-0.1-SNAPSHOT.jar
 Required by:
 :git-plugin:unspecified
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