commit: don't change phases for preexisting commits I noticed when pulling with hg-git in a repository that already had the changes, but pulled from another Mercurial repository. This meant that hg-git would re-create exact matches of the changesets, and if they were public, they'd get reverted to drafts. This change fixes that behaviour by avoiding phase changes when: - the result isn't tip - the result wasn't hidden The latter check addresses some issues with the shelve test. To help identify this issue, I added a message that reports to the user that a commit didn't change anything, should they actually trigger this. Differential Revision: https://phab.mercurial-scm.org/D9253