Enable incrementals for easier testing and development

https://www.jenkins.io/doc/developer/tutorial-improve/enable-incrementals/
describes the steps taken in the commit.

https://www.jenkins.io/doc/developer/plugin-development/incrementals/
provides more details of the benefits of incrementals and the way that
they help users and developers.

.mvn/extensions.xml

@@ -0,0 +1,7 @@
+<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.0.0 http://maven.apache.org/xsd/core-extensions-1.0.0.xsd">
+  <extension>
+    <groupId>io.jenkins.tools.incrementals</groupId>
+    <artifactId>git-changelist-maven-extension</artifactId>
+    <version>1.8</version>
+  </extension>
+</extensions>

.mvn/maven.config

@@ -0,0 +1,2 @@
+-Pconsume-incrementals
+-Pmight-produce-incrementals

pom.xml

@@ -9,6 +9,9 @@
   </parent>
 
   <properties>
+    <revision>2.2.1</revision>
+    <changelist>-SNAPSHOT</changelist>
+    <gitHubRepo>jenkinsci/plot-plugin</gitHubRepo>
     <!-- Baseline Jenkins version you use to build the plugin. Users must have this version or newer to run. -->
     <jenkins.baseline>2.361</jenkins.baseline>
     <jenkins.version>${jenkins.baseline}.4</jenkins.version>
@@ -30,7 +33,7 @@
   <artifactId>plot</artifactId>
   <packaging>hpi</packaging>
   <name>Plot plugin</name>
-  <version>2.2.1-SNAPSHOT</version>
+  <version>${revision}${changelist}</version>
   <url>https://github.com/jenkinsci/plot-plugin</url>
 
   <developers>
@@ -140,10 +143,10 @@
   </pluginRepositories>
 
   <scm>
-    <connection>scm:git:https://github.com/jenkinsci/plot-plugin.git</connection>
-    <developerConnection>scm:git:[email protected]:jenkinsci/plot-plugin.git</developerConnection>
-    <url>https://github.com/jenkinsci/plot-plugin</url>
-    <tag>HEAD</tag>
+    <connection>scm:git:https://github.com/${gitHubRepo}.git</connection>
+    <developerConnection>scm:git:[email protected]:${gitHubRepo}.git</developerConnection>
+    <url>https://github.com/${gitHubRepo}</url>
+    <tag>${scmTag}</tag>
   </scm>
 
 </project>