git submodule add fails when using both --branch and --depth

[Thadeus Fleming <thadeus.j.fleming@gmail.com>]

I'm running git 2.14.2 on Ubuntu 16.04.

Compare the behavior of

> git clone --branch pu --depth 1 https://github.com/git/git git-pu

which clones only the latest commit of the pu branch and

> mkdir tmp && cd tmp && git init
> git submodule add --branch pu --depth 1 https://github.com/git/git \
  git-pu

which gives the error

fatal: 'origin/pu' is not a commit and a branch 'pu' cannot be created
from it
Unable to checkout submodule 'git-pu'

Investigating further, there is indeed only one commit in the local repo:

> cd git-pu
> git log --oneline | wc -l
1

But that commit is the head of master.

> git branch -a
* master                                 remotes/origin/master
  remotes/origin/HEAD -> origin/master

This appears to be because git-submodule--helper does not accept a
--branch option. Using the --depth N option causes it to only clone N
commits from the default branch, which generally do not include the
desired branch. Thus, the next step,

git checkout -f -q -B "$branch" "origin/$branch"

fails, and provides the rather confusing error message above.

I'd suggest that git-submodule--helper learn a --branch option
consistent with git clone, and if that is impossible, that
git submodule add rejects the simultaneous use of both the --branch and
--depth options.



--tjf