absorb: clarify the reason for not finding changesets to modify

I'm used to pushing to non-publishing repos, so this was the last thing I
thought to check.

hgext/absorb.py

@@ -917,7 +917,7 b' def absorb(ui, repo, stack=None, targetc'
                       'be analysed\n')
                     % limit)
     if not stack:
-        raise error.Abort(_('no changeset to change'))
+        raise error.Abort(_('no mutable changeset to change'))
     if targetctx is None: # default to working copy
         targetctx = repo[None]
     if pats is None:

tests/test-absorb.t

@@ -18,7 +18,7 b''
 Do not crash with empty repo:
 
   $ hg absorb
-  abort: no changeset to change
+  abort: no mutable changeset to change
   [255]
 
 Make some commits:
@@ -225,7 +225,7 b' Merge commit will not be changed:'
   $ echo 2 >> m1
   $ echo 2 >> m2
   $ hg absorb
-  abort: no changeset to change
+  abort: no mutable changeset to change
   [255]
   $ hg revert -q -C m1 m2