improve error when unable to reverse adjust merge

Give the user a hint about what to do, as well as what they should have
done to avoid the problem.

diff --git a/Annex/AdjustedBranch.hs b/Annex/AdjustedBranch.hs
index 4f72cbd979d90d3c49087f92df0836cdde005762..e5f7359c95e865817ac18199696fbe5003fc5082 100644 (file)
--- a/Annex/AdjustedBranch.hs
+++ b/Annex/AdjustedBranch.hs
@@ -579,8 +579,7 @@ rebaseOnTopMsg = "rebasing adjusted branch on top of updated original branch"
  - will be lost, and any other headers are not copied either. -}
 reverseAdjustedCommit :: Sha -> Adjustment -> (Sha, Commit) -> OrigBranch -> Annex Sha
 reverseAdjustedCommit commitparent adj (csha, basiscommit) origbranch
-       | length (commitParent basiscommit) > 1 = giveup $
-               "unable to propagate merge commit " ++ show csha ++ " back to " ++ show origbranch
+       | length (commitParent basiscommit) > 1 = giveup mergeerror
        | otherwise = do
                cmode <- annexCommitMode <$> Annex.getGitConfig
                treesha <- reverseAdjustedTree commitparent adj csha
@@ -591,6 +590,12 @@ reverseAdjustedCommit commitparent adj (csha, basiscommit) origbranch
                                        [commitMessage basiscommit]
                                        [commitparent] treesha
                return revadjcommit
+  where
+       mergeerror = "Unable to propagate merge commit " ++ fromRef csha ++
+               " back to " ++ fromRef origbranch ++
+               " from this adjusted branch." ++
+               " To fix this, reset back to before the merge commit, " ++
+               " and use: git-annex merge <branch>"
 
 {- Adjusts the tree of the basis, changing only the files that the
  - commit changed, and reverse adjusting those changes.