git-rebase--interactive needs a better message

[Dmitry Potapov <dpotapov@gmail.com>]

I have tried to use git-rebase --interactive today, and run into a strange
error message saying:

/usr/bin/git-rebase--interactive: line 333: $GIT_DIR/.dotest-merge/author-script: No such file or directory

I had to scratch my head for a while before I realized that I forgot to
say git-commit. So, it was mine mistake, but I think that it should be
possible to have a better error message suggesting the user what to do.
I have looked at the git-rebase--interactive script, but I am not sure
that I understand it good enough to propose a patch.

To reproduce the problem, run the following script in an empty directory:
===
#!/bin/sh
set -e

git-init
echo 'version 1' > foo
git-add foo
git-commit -m 'commit 1' foo
echo 'versionn 2' >> foo
git-commit -m 'commit 2' foo
echo 'version 3' >> foo
git-commit -m 'commit 3' foo

GIT_EDITOR='sed -i -e "/commit 2/{s/^pick/edit/}"' git-rebase -i HEAD~2
sed -i -e "s/versionn/version/" foo
git-update-index foo
# Missing git-commit --amend
git-rebase --continue
===

Error message:
==
/usr/bin/git-rebase--interactive: line 333: /tmp/zzz/.git/.dotest-merge/author-script: No such file or directory
===

I use git version 1.5.3.1

Dmitry Potapov