git-build-recipe 'nest' fails if directory exists (bzr-builder works)

[Scott Moser]

# fresh bionic container, as root.
$ apt-get update && apt-get install git-build-recipe bzr-builder

$ bzr branch lp:simplestreams trunk.dist
# 460 is the last functional bzr version
$ bzr branch -r 460 trunk.dist upstream-bzr
$ bzr branch lp:~simplestreams-dev/simplestreams/pkg-trusty pkg-trusty
$ git clone https://git.launchpad.net/simplestreams upstream-git

$ cat recipe-bzr
# bzr-builder format 0.3 deb-version 0.1.0~bzr{revno}-0ubuntu1~14.04.{revno:packaging}
./upstream-bzr
nest packaging ./pkg-trusty debian

$ cat recipe-git
# git-build-recipe format 0.4 deb-version {latest-tag}-{revno}-g{git-commit}-0ubuntu1
./upstream-git master
nest packaging ./upstream-git debian ubuntu/trusty-packaging-only

$ cat run-bzr
#!/bin/sh
bzr -Derror dailydeb --safe --no-build \
    --manifest=manifest-bzr.txt \
    --distribution=trusty --allow-fallback-to-native \
    --append-version=~ubuntu14.04.1 ./recipe-bzr ./worktree-bzr

$ cat run-git
#!/bin/sh
git-build-recipe --safe --no-build \
    --manifest=manifest-git \
    --distribution=trusty --allow-fallback-to-native \
    --append-version=~ubuntu14.04.1 ./recipe-git ./worktree-git/

$ ./run-bzr
Building tree.
Retrieving './upstream-bzr' to put at './worktree-bzr/recipe-bzr-0.1.0~bzr{revno}-0ubuntu1~14.04.{revno:packaging}'.
Retrieving './pkg-trusty' to put at './worktree-bzr/recipe-bzr-0.1.0~bzr{revno}-0ubuntu1~14.04.{revno:packaging}/debian'.

$ ./run-git
Traceback (most recent call last):
  File "/usr/bin/git-build-recipe", line 11, in <module>
    load_entry_point('git-build-recipe==0.1', 'console_scripts', 'git-build-recipe')()
  File "/usr/lib/python3/dist-packages/gitbuildrecipe/main.py", line 110, in main
    build_tree(base_branch, working_directory)
  File "/usr/lib/python3/dist-packages/gitbuildrecipe/recipe.py", line 479, in build_tree
    _build_inner_tree(base_branch, target_path)
  File "/usr/lib/python3/dist-packages/gitbuildrecipe/recipe.py", line 403, in _build_inner_tree
    instruction.apply(target_path)
  File "/usr/lib/python3/dist-packages/gitbuildrecipe/recipe.py", line 582, in apply
    target_path=os.path.join(target_path, self.nest_path))
  File "/usr/lib/python3/dist-packages/gitbuildrecipe/recipe.py", line 400, in _build_inner_tree
    pull_or_clone(base_branch, target_path)
  File "/usr/lib/python3/dist-packages/gitbuildrecipe/recipe.py", line 306, in pull_or_clone
    raise TargetAlreadyExists(target_path)
gitbuildrecipe.recipe.TargetAlreadyExists: ./worktree-git/recipe-git/debian already exists, but has no .git directory.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: git-build-recipe 0.3.5
ProcVersionSignature: Ubuntu 4.17.0-6.7-generic 4.17.9
Uname: Linux 4.17.0-6-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
Date: Mon Aug 20 20:12:59 2018
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
SourcePackage: git-build-recipe
UpgradeStatus: No upgrade log present (probably fresh install)