[JENKINS-26100] Supply GitSCM/buildEnv.

src/main/java/hudson/plugins/git/GitSCM.java

@@ -1286,6 +1286,7 @@ public void buildEnvironment(Run<?, ?> build, java.util.Map<String, String> env)
               String remoteBranchName = getBranchName(branch);
                env.put(GIT_BRANCH, remoteBranchName);

                // TODO this is unmodular; should rather override LocalBranch.populateEnvironmentVariables
                LocalBranch lb = getExtensions().get(LocalBranch.class);
                if (lb != null) {
                   // Set GIT_LOCAL_BRANCH variable from the LocalBranch extension

src/main/resources/hudson/plugins/git/GitSCM/buildEnv.groovy

@@ -0,0 +1,10 @@
package hudson.plugins.git.GitSCM;

def l = namespace(lib.JenkinsTagLib)

// TODO handle GitSCMExtension.populateEnvironmentVariables somehow, say by optionally including GitSCMExtension/buildEnv.groovy; though GIT_{COMMITTER,AUTHOR}_{NAME,EMAIL} are only overridden by UserIdentity
['GIT_COMMIT', 'GIT_PREVIOUS_COMMIT', 'GIT_PREVIOUS_SUCCESSFUL_COMMIT', 'GIT_BRANCH', 'GIT_LOCAL_BRANCH', 'GIT_URL', 'GIT_COMMITTER_NAME', 'GIT_AUTHOR_NAME', 'GIT_COMMITTER_EMAIL', 'GIT_AUTHOR_EMAIL'].each {name ->
    l.buildEnvVar(name: name) {
        raw(_("${name}.blurb"))
    }
}

src/main/resources/hudson/plugins/git/GitSCM/buildEnv.properties

@@ -0,0 +1,10 @@
GIT_COMMIT.blurb=The commit hash being checked out.
GIT_PREVIOUS_COMMIT.blurb=The hash of the commit last built on this branch, if any.
GIT_PREVIOUS_SUCCESSFUL_COMMIT.blurb=The hash of the commit last successfully built on this branch, if any.
GIT_BRANCH.blurb=The remote branch name, if any.
GIT_LOCAL_BRANCH.blurb=The local branch name being checked out, if applicable.
GIT_URL.blurb=The remote URL. If there are multiple, will be <code>GIT_URL_1</code>, <code>GIT_URL_2</code>, etc.
GIT_COMMITTER_NAME.blurb=The configured Git committer name, if any.
GIT_AUTHOR_NAME.blurb=The configured Git author name, if any.
GIT_COMMITTER_EMAIL.blurb=The configured Git committer email, if any.
GIT_AUTHOR_EMAIL.blurb=The configured Git author email, if any.