Change default PR build strategy from merge to head (#636)

Author: jglick

src/main/java/org/jenkinsci/plugins/github_branch_source/GitHubSCMSource.java

@@ -2522,9 +2522,9 @@ public List<NamedArrayList<? extends SCMTraitDescriptor<?>>> getTraitsDescriptor
     public List<SCMSourceTrait> getTraitsDefaults() {
       return Arrays.asList( // TODO finalize
           new BranchDiscoveryTrait(true, false),
-          new OriginPullRequestDiscoveryTrait(EnumSet.of(ChangeRequestCheckoutStrategy.MERGE)),
+          new OriginPullRequestDiscoveryTrait(EnumSet.of(ChangeRequestCheckoutStrategy.HEAD)),
           new ForkPullRequestDiscoveryTrait(
-              EnumSet.of(ChangeRequestCheckoutStrategy.MERGE),
+              EnumSet.of(ChangeRequestCheckoutStrategy.HEAD),
               new ForkPullRequestDiscoveryTrait.TrustPermission()));
     }
 

src/main/resources/org/jenkinsci/plugins/github_branch_source/ForkPullRequestDiscoveryTrait/help-strategyId.html

@@ -4,6 +4,7 @@
         <dt>Merging the pull request with the current target branch revision</dt>
         <dd>Discover each pull request once with the discovered revision corresponding to the result of merging with the
             current revision of the target branch.
+            Note that pushes to the target branch will result in new pull request builds.
         </dd>
         <dt>The current pull request revision</dt>
         <dd>Discover each pull request once with the discovered revision corresponding to the pull request head revision

src/main/resources/org/jenkinsci/plugins/github_branch_source/OriginPullRequestDiscoveryTrait/help-strategyId.html

@@ -4,6 +4,7 @@
         <dt>Merging the pull request with the current target branch revision</dt>
         <dd>Discover each pull request once with the discovered revision corresponding to the result of merging with the
             current revision of the target branch.
+            Note that pushes to the target branch will result in new pull request builds.
         </dd>
         <dt>The current pull request revision</dt>
         <dd>Discover each pull request once with the discovered revision corresponding to the pull request head revision