destmerge: extract logic based on bookmark into 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.

mercurial/destutil.py

@@ -131,8 +131,9 b' def destupdate(repo, clean=False, check='
 
     return rev, movemark, activemark
 
-def destmerge(repo):
-    if repo._activebookmark:
+def _destmergebook(repo):
+    """find merge destination in the active bookmark case"""
+    node = None
         bmheads = repo.bookmarkheads(repo._activebookmark)
         curhead = repo[repo._activebookmark].node()
         if len(bmheads) == 2:
@@ -148,6 +149,12 b' def destmerge(repo):'
             raise error.Abort(_("no matching bookmark to merge - "
                 "please merge with an explicit rev or bookmark"),
                 hint=_("run 'hg heads' to see all heads"))
+    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)