Add support for merge-friendly branches

Add support for merge-friendly branches

The main issue with publishing StGit branches is that the Git history
represented by patches is volatile, making it difficult for people
wanting to merge such branch. One solution is for all the downstream
developers to always rebase but that's not always desirable. Another
solution is provided by tools like TopGit but the visible Git history
becomes complicated, especially with repeated reordering.

The patch proposes a new StGit command called "publish". This command
allows one to develop patches normally on a StGit branch but publish the
stack changes to a separate, merge-friendly branch whose history is not
re-writable.

More about its behaviour can be found in the command description in this
patch.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>