merge: ensure that we always commit the mergestate

In future patches, we may halt the merge process based on configuration or
user requests by raising exceptions. We need to ensure that the mergestate
is unconditionally committed even when such an exception is raised.

Depends on D930.

Differential Revision: https://phab.mercurial-scm.org/D931

mercurial/merge.py

@@ -1537,6 +1537,7 def applyupdates(repo, actions, wctx, mc
                 newactions.append((f, args, msg))
         mergeactions = newactions
 
+    try:
     # premerge
     tocomplete = []
     for f, args, msg in mergeactions:
@@ -1560,6 +1561,7 def applyupdates(repo, actions, wctx, mc
         progress(_updating, z, item=f, total=numupdates, unit=_files)
         ms.resolve(f, wctx)
 
+    finally:
     ms.commit()
 
     unresolved = ms.unresolvedcount()