precheck: fix false warning about content-divergence creation

Before this patch, if we try to `hg prune` (without any successors) an
already obsoleted cset which has at least one successor, it would false
warn about new content-divergence. As we know, pruning cset without any
successors can not create any divergence.

Differential Revision: https://phab.mercurial-scm.org/D12002

mercurial/rewriteutil.py

@@ -49,10 +49,13 b' def _formatrevs(repo, revs, maxrevs=4):'
     return summary
 
 
-def precheck(repo, revs, action=b'rewrite'):
+def precheck(repo, revs, action=b'rewrite', check_divergence=True):
     """check if revs can be rewritten
     action is used to control the error message.
 
+    check_divergence allows skipping the divergence checks in cases like adding
+    a prune marker (A, ()) to obsstore (which can't be diverging).
+
     Make sure this function is called after taking the lock.
     """
     if nullrev in revs:
@@ -85,6 +88,9 b" def precheck(repo, revs, action=b'rewrit"
             hint=hint,
         )
 
+    if not check_divergence:
+        return
+
     if not obsolete.isenabled(repo, obsolete.allowdivergenceopt):
         new_divergence = _find_new_divergence(repo, revs)
         if new_divergence: