"pipenv lock --requirements" leaks output from git, making the result invalid, only the first time

Issue description

One of my dependencies is in the format "lib" = {ref = "branch-name", git = "https://url/to/repo.git"}.

I run pipenv lock --requirements > requirements.txt for the first time inside a docker container.

Expected result

I get a valid requirements.txt

Actual result

See below

Steps to replicate

docker run -it --rm python:3.6 bash

Inside container

pip install pipenv==2018.11.26

echo '                        
[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[dev-packages]

[packages]
"numpy" = {ref = "maintenance/1.15.x", git = "https://github.com/numpy/numpy.git"}
' > Pipfile

pipenv lock --requirements > requirements.txt

Resultant requirements.txt (yes really):

Switched to a new branch 'maintenance/1.15.x'
Branch maintenance/1.15.x set up to track remote branch maintenance/1.15.x from origin.
-i https://pypi.org/simple
git+https://github.com/numpy/numpy.git@3430d78c01a3b9a19adad75f1acb5ae18286da73#egg=numpy

And of course:

root@3f46e6fed2ba:/# pip install -r requirements.txt 
Invalid requirement: 'Switched to a new branch 'maintenance/1.15.x''
It looks like a path. File 'Switched to a new branch 'maintenance/1.15.x'' does not exist.

Workaround

I run pipenv lock --requirements > /dev/null 2>&1 once before running the actual command.

---

Please run $ pipenv --support, and paste the results here. Don’t put backticks (`) around it! The output already contains Markdown formatting.

$ pipenv --support

Pipenv version: '2018.11.26'

Pipenv location: '/usr/local/lib/python3.6/site-packages/pipenv'

Python location: '/usr/local/bin/python'

Python installations found:

• 3.6.7: /usr/local/bin/python
• 3.6.7: /usr/local/bin/python3.6m
• 3.5.3: /usr/bin/python3
• 3.5.3: /usr/bin/python3.5m
• 2.7.13: /usr/bin/python2.7

PEP 508 Information:

{'implementation_name': 'cpython',
 'implementation_version': '3.6.7',
 'os_name': 'posix',
 'platform_machine': 'x86_64',
 'platform_python_implementation': 'CPython',
 'platform_release': '4.15.0-39-generic',
 'platform_system': 'Linux',
 'platform_version': '#42~16.04.1-Ubuntu SMP Wed Oct 24 17:09:54 UTC 2018',
 'python_full_version': '3.6.7',
 'python_version': '3.6',
 'sys_platform': 'linux'}

System environment variables:

• LANG
• HOSTNAME
• GPG_KEY
• PWD
• HOME
• TERM
• PYTHON_VERSION
• SHLVL
• PATH
• PYTHON_PIP_VERSION
• _
• PIP_DISABLE_PIP_VERSION_CHECK
• PYTHONDONTWRITEBYTECODE
• PIP_SHIMS_BASE_MODULE
• PIP_PYTHON_PATH
• PYTHONFINDER_IGNORE_UNSUPPORTED

Pipenv–specific environment variables:

Debug–specific environment variables:

• PATH: /usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
• LANG: C.UTF-8
• PWD: /

---

Contents of Pipfile (‘/Pipfile’):

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[dev-packages]

[packages]
"numpy" = {ref = "maintenance/1.15.x", git = "https://github.com/numpy/numpy.git"}

Contents of Pipfile.lock (‘/Pipfile.lock’):

{
    "_meta": {
        "hash": {
            "sha256": "e8d89524a159e6d44fa849ffd5e1bf02b13a73b773013996d77d5405776369eb"
        },
        "pipfile-spec": 6,
        "requires": {},
        "sources": [
            {
                "name": "pypi",
                "url": "https://pypi.org/simple",
                "verify_ssl": true
            }
        ]
    },
    "default": {
        "numpy": {
            "git": "https://github.com/numpy/numpy.git",
            "ref": "3430d78c01a3b9a19adad75f1acb5ae18286da73"
        }
    },
    "develop": {}
}

[jagguli]

still exists with version pipenv-2018.11.26

[techalchemy]

Awesome, thanks @cjerdonek — I merged some changes to requirementslib to monkeypatch pip and capture stdout in the meantime