revlog-compression: fix computation of engine availability

We don't just need the engine to be define, we need it to be available and able
to do be used for revlog compression. Without this change, `zstd` could be
selected as a viable option for repository creation on platform where it is not
available.

Differential Revision: https://phab.mercurial-scm.org/D10325

mercurial/localrepo.py

@@ -3470,7 +3470,9 b' def newreporequirements(ui, createopts):'
     compengines = ui.configlist(b'format', b'revlog-compression')
     for compengine in compengines:
         if compengine in util.compengines:
-            break
+            engine = util.compengines[compengine]
+            if engine.available() and engine.revlogheader():
+                break
     else:
         raise error.Abort(
             _(

mercurial/upgrade_utils/actions.py

@@ -428,7 +428,9 b' class compressionengine(formatvariant):'
         # return the first valid value as the selection code would do
         for comp in compengines:
             if comp in util.compengines:
-                return comp
+                e = util.compengines[comp]
+                if e.available() and e.revlogheader():
+                    return comp
 
         # no valide compression found lets display it all for clarity
         return b','.join(compengines)