rewriteutil: add a precheck function to check if revs can be rewritten
The precheck function is intended to be used before we start rewritting
changesets.
Differential Revision: https://phab.mercurial-scm.org/D1503
rewriteutil: add utility function to check if we can create new unstable cset
This patch adds a new file which will contain utility functions related to
rewritting changesets. It also adds a utility function to check if the
rewritting operation creates new unstable changesets and are we allowed to
create them.
This rewriteutil.py introduced in this patch and the utility functions added in
the upcoming patches exists in the evolve extension are being ported from there.
Differential Revision: https://phab.mercurial-scm.org/D1502