fix: use obsolete.isenabled() to check for experimental.allowdivergence stable

fix: use obsolete.isenabled() to check for experimental.allowdivergence

Now that obsolete.isenabled() can also check if divergence is allowed, let's
use it for consistency. Other experimental.evolution options are already
checked via this function.

Differential Revision: https://phab.mercurial-scm.org/D11222

--- a/hgext/fix.py	Wed Jul 28 13:45:41 2021 +0300
+++ b/hgext/fix.py	Wed Jul 28 13:47:21 2021 +0300
@@ -148,6 +148,7 @@
     mdiff,
     merge,
     mergestate as mergestatemod,
+    obsolete,
     pycompat,
     registrar,
     rewriteutil,
@@ -452,10 +453,7 @@
     """Aborts if the revision shouldn't be replaced with a fixed one."""
     if ctx.obsolete():
         # It would be better to actually check if the revision has a successor.
-        allowdivergence = ui.configbool(
-            b'experimental', b'evolution.allowdivergence'
-        )
-        if not allowdivergence:
+        if not obsolete.isenabled(repo, obsolete.allowdivergenceopt):
             raise error.Abort(
                 b'fixing obsolete revision could cause divergence'
             )