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
3 files changed