RFE: rdopkg amend should preserve Change-Id

[yac]

Running rdopkg amend && git review results in new Change-Id and new review which is inconvenient.

Changes to happen in rdopkg.actions.distgit.actions module:

• Add amend=False parameter to _commit_message() and reuse last commit message Change-Id if amend==True there. Make this extensible in a way so that more information can be reused from last commit message if needed in the future (Fixes, rhbz, ...?)
• Pass amend parameter to _commit_message() call in commit_distgit_update()
• Pass amend=True to _commit_message() call in amend()

[yac]

It makes sense to solve this after #147 .

[yac]

Solved in 1fd6fad45b .