commit: reorder if-else conditional to give mergestate info priority

Looking at the code now, I was unable to find a good reason as why we only rely
on mergestate extras info after checking whether a filelog parent is ancestor of
other or not.

I mean if we have stored in mergestate that `other` was chosed, we should
blindly pick that one.

This cleanup will also help introduce more cases when both `fparent1` and
`fparent2` are non-null but using info from mergestate, we can fastpath.

Differential Revision: https://phab.mercurial-scm.org/D10116

mercurial/commit.py

@@ -359,19 +359,18 b' def _filecommit('
     elif fparent1 == nullid:
         fparent1, fparent2 = fparent2, nullid
     elif fparent2 != nullid:
-        # is one parent an ancestor of the other?
-        fparentancestors = flog.commonancestorsheads(fparent1, fparent2)
-        if fparent1 in fparentancestors:
+        if (
+            ms.active()
+            and ms.extras(fname).get(b'filenode-source') == b'other'
+        ):
             fparent1, fparent2 = fparent2, nullid
-        elif fparent2 in fparentancestors:
-            fparent2 = nullid
-        elif not fparentancestors:
-            # TODO: this whole if-else might be simplified much more
-            if (
-                ms.active()
-                and ms.extras(fname).get(b'filenode-source') == b'other'
-            ):
+        # is one parent an ancestor of the other?
+        else:
+            fparentancestors = flog.commonancestorsheads(fparent1, fparent2)
+            if fparent1 in fparentancestors:
                 fparent1, fparent2 = fparent2, nullid
+            elif fparent2 in fparentancestors:
+                fparent2 = nullid
 
     force_new_node = False
     # The file might have been deleted by merge code and user explicitly choose