Add "stg coalesce"

It coalesces two or more consecutive applied patches, with no need to
touch index/worktree, and no possibiliy of conflicts.

Future improvements could relax the "consecutive" and "applied"
restrictions, by building a new chain of commits just like "stg push"
will do once it's been converted to the new infrastructure.

Signed-off-by: Karl Hasselström <kha@treskal.com>