Regression `git checkout $rev -b branch` while in a `--no-checkout` clone does not check out files

[Anthony Sottile <asottile@xxxxxxxxx>]

Here's a simple regression test -- haven't had time to bisect this

```
#!/usr/bin/env bash
set -euxo pipefail

rm -rf src dest

git --version

git init src
echo hi > src/a
git -C src add .
git -C src commit -m "initial commit"
rev="$(git -C src rev-parse HEAD)"

git clone --no-checkout src dest
git -C dest checkout "$rev" -b branch
test -f dest/a

: 'SUCCESS!'
```

With git 2.17.1

```
+ set -euo pipefail
+ rm -rf src dest
+ git --version
git version 2.17.1
+ git init src
Initialized empty Git repository in /tmp/t/src/.git/
+ echo hi
+ git -C src add .
+ git -C src commit -m 'initial commit'
[master (root-commit) 61ae2ae] initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 a
++ git -C src rev-parse HEAD
+ rev=61ae2ae9c9a96de7b1688b095f20a6adf9c20db1
+ git clone --no-checkout src dest
Cloning into 'dest'...
done.
+ git -C dest checkout 61ae2ae9c9a96de7b1688b095f20a6adf9c20db1 -b branch
Switched to a new branch 'branch'
+ test -f dest/a
+ : 'SUCCESS!'
```

With git 2.20.GIT (b21ebb671bb7dea8d342225f0d66c41f4e54d5ca)

```
+ set -euo pipefail
+ rm -rf src dest
+ git --version
git version 2.20.GIT
+ git init src
Initialized empty Git repository in /tmp/t/src/.git/
+ echo hi
+ git -C src add .
+ git -C src commit -m 'initial commit'
[master (root-commit) df4d6dc] initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 a
++ git -C src rev-parse HEAD
+ rev=df4d6dcf02b15bfe2b3f0e4a8aa25ac165b1368c
+ git clone --no-checkout src dest
Cloning into 'dest'...
done.
+ git -C dest checkout df4d6dcf02b15bfe2b3f0e4a8aa25ac165b1368c -b branch
D    a
Switched to a new branch 'branch'
+ test -f dest/a
```

A workaround for my use case is to not use `--no-checkout`, though
this is potentially slow

Anthony