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

test-run-tests: do not rebuild hg in the test d600bda4 and fc0f3ed0 added code to call `$PYTHON run-tests.py ...`. That will rebuild hg, is slow and could have other crashes from setup.py, like: Unable to find a working hg binary to extract the version from the repository tags Therefore use `run-tests.py -l` instead. Differential Revision: https://phab.mercurial-scm.org/D1595

remotenames: consider existing data while storing newer data Previously reviewed as D1357. Differential Revision: https://phab.mercurial-scm.org/D1551