Bazel: Support building with Java 9

Bazel@HEAD supports Java 9.

The current code has one single issue with Java 9 compliance: the usage
of javax.xml.bind.DatatypeConverter class for printHexBinary() method.
This class is not available on Java 9. One alternative is to use guava
library. Something similar was done here: [1]. But unlike the case with
checkstyle library, JGit currently doesn't use guava. Instead, we add
java.xml.bind module with --add-modules compiler option.

To build (or test) with Java 9, build custom bazel version and issue:

  $ bazel --host_javabase=/usr/lib64/jvm/java-9-openjdk build \
    --javacopt='--release 9' \
    --java_toolchain=@bazel_tools//tools/jdk:toolchain_jdk9 :all

The Java 9 support is backwards compatible.

* [1] https://github.com/checkstyle/checkstyle/issues/5027

Change-Id: I2c5203fc4e65885ce7b210f824fda85ba6d6c51d
Signed-off-by: David Ostrovsky <david@ostrovsky.org>

BUILD

@@ -1,5 +1,12 @@
 package(default_visibility = ["//visibility:public"])
 
+config_setting(
+    name = "jdk9",
+    values = {
+        "java_toolchain": "@bazel_tools//tools/jdk:toolchain_jdk9",
+    },
+)
+
 genrule(
     name = "all",
     testonly = 1,

org.eclipse.jgit/BUILD

@@ -14,6 +14,10 @@ RESOURCES = glob(["resources/**"])
 java_library(
     name = "jgit",
     srcs = SRCS,
+    javacopts = select({
+        "//:jdk9": ["--add-modules=java.xml.bind"],
+        "//conditions:default": [],
+    }),
     resource_strip_prefix = "org.eclipse.jgit/resources",
     resources = RESOURCES,
     deps = [