[BUG] Index.lock error message regression in git 2.11.0

[Robbie Iannucci <iannucci@google.com>]

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Hello,

I just upgraded to 2.11.0 from 2.10.2, and I noticed that some
commands no longer print an error message when the `index.lock` file
exists (such as `git merge --ff-only`).

It appears this bug was introduced in
55f5704da69d3e6836620f01bee0093ad5e331e8 (sequencer: lib'ify
checkout_fast_forward()). I determined this by running the attached
bisect script (on OS X, but I don't think that matters; I've also seen
the error message missing on Linux):

$ cd /path/to/git/src
$ git bisect start v2.11.0-rc0 v2.10.2
$ git bisect run /path/to/bisect.test.sh

(my original version of the script also includes some other makefile
parameters to get a modern version of gettext and openssl too, but
they're not relevant to this ML).

I'm not certain that I have enough context to propose a meaningful
patch though :/.

Cheers,
Robbie

[-- Attachment #2: bisect.test.sh --]
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#!/bin/bash

make -j 8 2>&1 > /dev/null
if [[ $? != 0 ]]; then
  # skip this version
  exit 125
fi
git=`realpath ./git`

rm -rf .test_repo || true
mkdir .test_repo

cd .test_repo
$git init

echo HELLO > a
$git add a
$git commit -m 'initial_commit'
$git branch --track new_branch

$git checkout master
echo HELLO >> a
$git add a
$git commit -m 'second_commit'

$git checkout new_branch

touch .git/index.lock
if $git merge --ff-only master 2>&1 | grep -F "index.lock" ; then
  echo OK
  exit 0
fi
echo Message is missing
exit 1