destutil: extract all 'mergedest' abort messages into a dictionary

destutil: extract all 'mergedest' abort messages into a dictionary

We plan to be able to reuse this function for rebase. The error
message explicitly refers to "merge" in multiple places. So we'll need
to be able to use different messages. The first step of that is to
extract all messages in a dedicated dictionary and use them
indirectly.

As a side effect it clarifies the actual function and opens the way to
various cleanups and fixes in future changesets.

--- a/mercurial/destutil.py	Mon Feb 08 14:03:45 2016 +0100
+++ b/mercurial/destutil.py	Tue Feb 09 21:14:37 2016 +0000
@@ -134,6 +134,43 @@
 
     return rev, movemark, activemark
 
+msgdestmerge = {
+    # too many matching divergent bookmark
+    'toomanybookmarks':
+        (_("multiple matching bookmarks to merge -"
+           " please merge with an explicit rev or bookmark"),
+         _("run 'hg heads' to see all heads")),
+    # no other matching divergent bookmark
+    'nootherbookmarks':
+        (_("no matching bookmark to merge - "
+           "please merge with an explicit rev or bookmark"),
+         _("run 'hg heads' to see all heads")),
+    # branch have too many unbookmarked heads, no obvious destination
+    'toomanyheads':
+        (_("branch '%s' has %d heads - please merge with an explicit rev"),
+         _("run 'hg heads .' to see heads")),
+    # branch have no other unbookmarked heads
+    'bookmarkedheads':
+        (_("heads are bookmarked - please merge with an explicit rev"),
+         _("run 'hg heads' to see all heads")),
+    # branch have just a single heads, but there is other branches
+    'nootherbranchheads':
+        (_("branch '%s' has one head - please merge with an explicit rev"),
+         _("run 'hg heads' to see all heads")),
+    # repository have a single head
+    'nootherheads':
+        (_('nothing to merge'),
+         None),
+    # repository have a single head and we are not on it
+    'nootherheadsbehind':
+        (_('nothing to merge'),
+         _("use 'hg update' instead")),
+    # We are not on a head
+    'notatheads':
+        (_('working directory not at a head revision'),
+         _("use 'hg update' or merge with an explicit revision"))
+        }
+
 def _destmergebook(repo):
     """find merge destination in the active bookmark case"""
     node = None
@@ -145,13 +182,11 @@
         else:
             node = bmheads[0]
     elif len(bmheads) > 2:
-        raise error.Abort(_("multiple matching bookmarks to merge - "
-            "please merge with an explicit rev or bookmark"),
-            hint=_("run 'hg heads' to see all heads"))
+        msg, hint = msgdestmerge['toomanybookmarks']
+        raise error.Abort(msg, hint=hint)
     elif len(bmheads) <= 1:
-        raise error.Abort(_("no matching bookmark to merge - "
-            "please merge with an explicit rev or bookmark"),
-            hint=_("run 'hg heads' to see all heads"))
+        msg, hint = msgdestmerge['nootherbookmarks']
+        raise error.Abort(msg, hint=hint)
     assert node is not None
     return node
 
@@ -163,31 +198,26 @@
     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"))
+        msg, hint = msgdestmerge['toomanyheads']
+        msg %= (branch, len(bheads))
+        raise error.Abort(msg, hint=hint)
 
     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")
+            msg, hint = msgdestmerge['bookmarkedheads']
+        elif len(repo.heads()) > 1:
+            msg, hint = msgdestmerge['nootherbranchheads']
+            msg %= branch
+        elif parent != repo.lookup(branch):
+            msg, hint = msgdestmerge['nootherheadsbehind']
+        else:
+            msg, hint = msgdestmerge['nootherheads']
         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"))
+        msg, hint = msgdestmerge['notatheads']
+        raise error.Abort(msg, hint=hint)
     if parent == nbhs[0]:
         node = nbhs[-1]
     else: