destutil: add the ability to specify a search space for rebase destination
In the 'hg pull --rebase', we don't want to pick a rebase destination unrelated
to the pull, we lay down basic infrastructure to allow such restriction on
stable (before 3.8 release) in this case. See issue 5214 for details.
Actual usage and test will be in the next patch.
% creating repo
% add and commit
C test.py
% change
M test.py