Simplify local JGit development

Put all the logic in jgit.bzl, where a single edit suffices to get
the local flavor.

Given that all sha1 for jgit dependencies are in jgit.bzl, we can
remove the constants and use sha1 values directly.

Change-Id: Icabf651e02f226e5c025457d54588074a11ae283

lib/jgit/org.eclipse.jgit.junit/BUILD

@@ -1,11 +1,10 @@
+load("//lib/jgit:jgit.bzl", "jgit_dep")
+
 java_library(
     name = "junit",
     testonly = 1,
     data = ["//lib:LICENSE-DO_NOT_DISTRIBUTE"],
     visibility = ["//visibility:public"],
-    exports = select({
-        # "//lib/jgit:dev": ["@jgit//org.eclipse.jgit.junit:junit"],
-        "//conditions:default": ["@jgit_junit//jar"],
-    }),
+    exports = [jgit_dep("@jgit_junit//jar")],
     runtime_deps = ["//lib/jgit/org.eclipse.jgit:jgit"],
 )