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

hgext/fix.py

@@ -147,6 +147,7 b' from mercurial import ('
     mdiff,
     merge,
     mergestate as mergestatemod,
+    obsolete,
     pycompat,
     registrar,
     rewriteutil,
@@ -451,10 +452,7 b' def checkfixablectx(ui, repo, ctx):'
     """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'
             )