Factor out a common ObjectBuilder as super class of CommitBuilder
and TagBuilder, and make the GpgSigner work on ObjectBuilder.
In order not to break API, add the new method for signing an
ObjectBuilder in a new interface GpgObjectSigner.
The signature for a tag is just tacked onto the end of the tag
message. The message of a signed tag must end in LF.
Bug: 386908
Change-Id: I5e021e3c927f4051825cd7355b129113b949455e
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>
5abd8a4feb