Buck: extend the tool chain to support unsigning JARs

After switching to Eclipse Maven repository for pulling JGit lib, we
have the problem that according to Eclipse release train the JARs have
to be signed. That collids with our jgit patch for diff deserialization.

To rectify that, add `unsign` parameter to maven_jar() function.

Change-Id: Ib7bfa5d16f980a64b887d61a4b4ec325e6ffb0a1

lib/maven.defs

diff --git a/lib/maven.defs b/lib/maven.defs
index d49fed6..54840e8 100644
--- a/lib/maven.defs
+++ b/lib/maven.defs
@@ -33,6 +33,7 @@
     license,
     exclude = [],
     exclude_java_sources = False,
+    unsign = False,
     deps = [],
     sha1 = '', bin_sha1 = '', src_sha1 = '',
     repository = MAVEN_CENTRAL,
@@ -73,6 +74,8 @@
     cmd.extend(['-x', x])
   if exclude_java_sources:
     cmd.append('--exclude_java_sources')
+  if unsign:
+    cmd.append('--unsign')
 
   genrule(
     name = name + '__download_bin',