destmerge: extract logic based on branch heads in its own function

destmerge: extract logic based on branch heads in its own function

One of the main goal of having consolidated destination function is to allow
extension to play with this logic. We extract sub logic to make is wrapping more
practical.

--- a/mercurial/destutil.py	Thu Oct 15 03:13:14 2015 +0100
+++ b/mercurial/destutil.py	Thu Oct 15 03:15:54 2015 +0100
@@ -152,42 +152,49 @@
     assert node is not None
     return node
 
+def _destmergebranch(repo):
+    """find merge destination based on branch heads"""
+    node = None
+    branch = repo[None].branch()
+    bheads = repo.branchheads(branch)
+    nbhs = [bh for bh in bheads if not repo[bh].bookmarks()]
+
+    if len(nbhs) > 2:
+        raise error.Abort(_("branch '%s' has %d heads - "
+                           "please merge with an explicit rev")
+                         % (branch, len(bheads)),
+                         hint=_("run 'hg heads .' to see heads"))
+
+    parent = repo.dirstate.p1()
+    if len(nbhs) <= 1:
+        if len(bheads) > 1:
+            raise error.Abort(_("heads are bookmarked - "
+                               "please merge with an explicit rev"),
+                             hint=_("run 'hg heads' to see all heads"))
+        if len(repo.heads()) > 1:
+            raise error.Abort(_("branch '%s' has one head - "
+                               "please merge with an explicit rev")
+                             % branch,
+                             hint=_("run 'hg heads' to see all heads"))
+        msg, hint = _('nothing to merge'), None
+        if parent != repo.lookup(branch):
+            hint = _("use 'hg update' instead")
+        raise error.Abort(msg, hint=hint)
+
+    if parent not in bheads:
+        raise error.Abort(_('working directory not at a head revision'),
+                         hint=_("use 'hg update' or merge with an "
+                                "explicit revision"))
+    if parent == nbhs[0]:
+        node = nbhs[-1]
+    else:
+        node = nbhs[0]
+    assert node is not None
+    return node
+
 def destmerge(repo):
     if repo._activebookmark:
         node = _destmergebook(repo)
     else:
-        branch = repo[None].branch()
-        bheads = repo.branchheads(branch)
-        nbhs = [bh for bh in bheads if not repo[bh].bookmarks()]
-
-        if len(nbhs) > 2:
-            raise error.Abort(_("branch '%s' has %d heads - "
-                               "please merge with an explicit rev")
-                             % (branch, len(bheads)),
-                             hint=_("run 'hg heads .' to see heads"))
-
-        parent = repo.dirstate.p1()
-        if len(nbhs) <= 1:
-            if len(bheads) > 1:
-                raise error.Abort(_("heads are bookmarked - "
-                                   "please merge with an explicit rev"),
-                                 hint=_("run 'hg heads' to see all heads"))
-            if len(repo.heads()) > 1:
-                raise error.Abort(_("branch '%s' has one head - "
-                                   "please merge with an explicit rev")
-                                 % branch,
-                                 hint=_("run 'hg heads' to see all heads"))
-            msg, hint = _('nothing to merge'), None
-            if parent != repo.lookup(branch):
-                hint = _("use 'hg update' instead")
-            raise error.Abort(msg, hint=hint)
-
-        if parent not in bheads:
-            raise error.Abort(_('working directory not at a head revision'),
-                             hint=_("use 'hg update' or merge with an "
-                                    "explicit revision"))
-        if parent == nbhs[0]:
-            node = nbhs[-1]
-        else:
-            node = nbhs[0]
+        node = _destmergebranch(repo)
     return repo[node].rev()