.gitignore: ignore all Maven output directories `target/`

Change-Id: Ib405f3db99290fe9c1f1349759f6598819f1b886
diff --git a/.gitignore b/.gitignore
index 5306e41..1671449 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,6 @@
 /.project
 /org.eclipse.jgit.benchmarks/dependency-reduced-pom.xml
-/target
+target/
 infer-out
 bazel-*