Provide a diff file to make it easier to deploy JGit snapshot

Provide a git diff that can be applied to JGit to make it easier to
apply the necessary changes in pom.xml for deployment to the Gerrit
Maven repository. Update the instructions to describe how to use it
as an alternative to manually updating pom.xml.

Change-Id: Ib8b2655abbd49b546e88ccf90def8329e749aa8b

tools/jgit-snapshot-deploy-pom.diff

@@ -0,0 +1,43 @@
+diff --git a/pom.xml b/pom.xml
+index d256bbb..7e523fd 100644
+--- a/pom.xml
++++ b/pom.xml
+@@ -226,6 +226,10 @@
+ 
+   <pluginRepositories>
+     <pluginRepository>
++      <id>gerrit-maven</id>
++      <url>https://gerrit-maven.commondatastorage.googleapis.com</url>
++    </pluginRepository>
++    <pluginRepository>
+       <id>repo.eclipse.org.cbi-releases</id>
+       <url>https://repo.eclipse.org/content/repositories/cbi-releases/</url>
+     </pluginRepository>
+@@ -236,6 +240,13 @@
+   </pluginRepositories>
+ 
+   <build>
++    <extensions>
++      <extension>
++        <groupId>com.googlesource.gerrit</groupId>
++        <artifactId>gs-maven-wagon</artifactId>
++        <version>3.3</version>
++      </extension>
++    </extensions>
+     <pluginManagement>
+       <plugins>
+         <plugin>
+@@ -649,9 +660,10 @@
+ 
+   <distributionManagement>
+     <repository>
+-      <id>repo.eclipse.org</id>
+-      <name>JGit Maven Repository - Releases</name>
+-      <url>https://repo.eclipse.org/content/repositories/jgit-releases/</url>
++      <id>gerrit-maven-repository</id>
++      <name>Gerrit Maven Repository</name>
++      <url>gs://gerrit-maven</url>
++      <uniqueVersion>true</uniqueVersion>
+     </repository>
+     <snapshotRepository>
+       <id>repo.eclipse.org</id>