Update JGit to 2.3.1.201302201838-r.63-gbba74ba

Includes NameRevCommand, which will be used to implement a "git
describe"-style API endpoint.

Change-Id: I2cae5e5951742f41abdc538a71cbc051334258ce

pom.xml

diff --git a/pom.xml b/pom.xml
index df7ccf8..b5d5399 100644
--- a/pom.xml
+++ b/pom.xml
@@ -32,7 +32,7 @@
 
   <properties>
     <!-- Should track Gerrit's jgitVersion fairly closely. -->
-    <jgitVersion>2.1.0.201209190230-r</jgitVersion>
+    <jgitVersion>2.3.1.201302201838-r.63-gbba74ba</jgitVersion>
     <jettyVersion>7.6.8.v20121106</jettyVersion>
     <slf4jVersion>1.6.1</slf4jVersion>
   </properties>