absorb: be more specific when erroring out on merge commit

When you have a merge commit checked out and run `hg absorb`, it would
tell you

  abort: no mutable changeset to change

That makes it sound like the problem is public commits when isn't
really. Let's be more specific in this case.

There was already a test case that test this, so that now prints the
new message. I added a new test case that shows the old message (when
a public commit is checked out).

Differential Revision: https://phab.mercurial-scm.org/D6354

hgext/absorb.py

@@ -914,7 +914,10 b' def absorb(ui, repo, stack=None, targetc'
     """
     if stack is None:
         limit = ui.configint('absorb', 'max-stack-size')
-        stack = getdraftstack(repo['.'], limit)
+        headctx = repo['.']
+        if len(headctx.parents()) > 1:
+            raise error.Abort(_('cannot absorb into a merge'))
+        stack = getdraftstack(headctx, limit)
         if limit and len(stack) >= limit:
             ui.warn(_('absorb: only the recent %d changesets will '
                       'be analysed\n')

tests/test-absorb.t

@@ -176,7 +176,7 b' Bookmarks are moved:'
      b2                        2:c9b20c925790
    * ba                        2:c9b20c925790
 
-Non-mofified files are ignored:
+Non-modified files are ignored:
 
   $ touch b
   $ hg commit -A b -m b
@@ -225,10 +225,15 b' Public commits will not be changed:'
   2: 4d
   2: insert aftert 4d
 
+  $ hg co -qC 1
+  $ sedi 's/Insert/insert/' a
+  $ hg absorb --apply-changes
+  abort: no mutable changeset to change
+  [255]
+
 Make working copy clean:
 
-  $ hg revert -q -C a b
-  $ hg forget c
+  $ hg co -qC ba
   $ rm c
   $ hg status
 
@@ -261,7 +266,7 b' Merge commit will not be changed:'
   $ echo 2 >> m1
   $ echo 2 >> m2
   $ hg absorb --apply-changes
-  abort: no mutable changeset to change
+  abort: cannot absorb into a merge
   [255]
   $ hg revert -q -C m1 m2