Generate reproducible build output (#243)

By deriving `project.build.outputTimestamp` from the timestamp of the 
most recent commit, the timestamp embedded in generated JARs no longer
depends on the exact time at which the artifacts are built. As such
repeated executions of `mvn clean install` yield byte-for-byte identical
results.

This change requires replacing `buildnumber-maven-plugin` with
`git-commit-id-maven-plugin`.

See https://maven.apache.org/guides/mini/guide-reproducible-builds.html

pom.xml

@@ -137,6 +137,9 @@
         property allows the `error-prone-fork` profile below to build the
         project using Picnic's Error Prone fork instead. -->
         <groupId.error-prone>com.google.errorprone</groupId.error-prone>
+        <!-- The build timestamp is derived from the most recent commit
+        timestamp in support of reproducible builds. -->
+        <project.build.outputTimestamp>${git.commit.time}</project.build.outputTimestamp>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <!-- Dependency and plugin versions that are referenced in more than
         one place. We use these to keep dependencies in sync. Version numbers
@@ -511,6 +514,25 @@
                         </execution>
                     </executions>
                 </plugin>
+                <plugin>
+                    <groupId>io.github.git-commit-id</groupId>
+                    <artifactId>git-commit-id-maven-plugin</artifactId>
+                    <version>5.0.0</version>
+                    <configuration>
+                        <injectAllReactorProjects>true</injectAllReactorProjects>
+                        <runOnlyOnce>true</runOnlyOnce>
+                        <skipPoms>false</skipPoms>
+                        <dateFormat>yyyy-MM-dd'T'HH:mm:ssXXX</dateFormat>
+                    </configuration>
+                    <executions>
+                        <execution>
+                            <id>retrieve-git-info</id>
+                            <goals>
+                                <goal>revision</goal>
+                            </goals>
+                        </execution>
+                    </executions>
+                </plugin>
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-checkstyle-plugin</artifactId>
@@ -968,9 +990,9 @@
                             <manifestEntries>
                                 <Implementation-Title>${project.name}</Implementation-Title>
                                 <Implementation-Vendor>${project.organization.name}</Implementation-Vendor>
-                                <Implementation-Version>${project.version}.${build.number}.${build.revision}</Implementation-Version>
+                                <Implementation-Version>${project.version}.${build.number}.${git.commit.id}</Implementation-Version>
                                 <Rookout-Repository>${project.scm.url}</Rookout-Repository>
-                                <Rookout-Revision>${build.revision}</Rookout-Revision>
+                                <Rookout-Revision>${git.commit.id}</Rookout-Revision>
                             </manifestEntries>
                         </archive>
                     </configuration>
@@ -1084,24 +1106,6 @@
                     <artifactId>build-helper-maven-plugin</artifactId>
                     <version>3.3.0</version>
                 </plugin>
-                <plugin>
-                    <groupId>org.codehaus.mojo</groupId>
-                    <artifactId>buildnumber-maven-plugin</artifactId>
-                    <version>3.0.0</version>
-                    <configuration>
-                        <buildNumberPropertyName>build.revision</buildNumberPropertyName>
-                        <revisionOnScmFailure>UNKNOWN</revisionOnScmFailure>
-                    </configuration>
-                    <executions>
-                        <execution>
-                            <id>determine-build-number</id>
-                            <goals>
-                                <goal>create</goal>
-                            </goals>
-                            <phase>validate</phase>
-                        </execution>
-                    </executions>
-                </plugin>
                 <plugin>
                     <groupId>org.codehaus.mojo</groupId>
                     <artifactId>license-maven-plugin</artifactId>
@@ -1284,12 +1288,12 @@
         </pluginManagement>
         <plugins>
             <plugin>
-                <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-source-plugin</artifactId>
+                <groupId>io.github.git-commit-id</groupId>
+                <artifactId>git-commit-id-maven-plugin</artifactId>
             </plugin>
             <plugin>
-                <groupId>org.codehaus.mojo</groupId>
-                <artifactId>buildnumber-maven-plugin</artifactId>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-source-plugin</artifactId>
             </plugin>
         </plugins>
     </build>