Update a specific package does not work.

[armenak-baburyan]

Update a specific package --upgrade-package or -P flag does not work.

Environment Versions

1. MacOS Sierra
2. Python version: 3.5.3
3. pip version: 9.0.1
4. pip-tools version: 1.8.0

Steps to replicate

req.in

jinja2
markupsafe
werkzeug

req.txt

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file requirements/req.txt requirements/req.in
#
jinja2==2.7.3
markupsafe==0.23
werkzeug==0.6

2. Trying to update only jinja2: pip-compile -P jinja2 requirements/req.in

Expected result

req.txt

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file requirements/req.txt requirements/req.in
#
jinja2==2.9.5
markupsafe==0.23
werkzeug==0.6

Actual result

req.txt

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file requirements/req.txt requirements/req.in
#
jinja2==2.9.5
markupsafe==0.23
werkzeug==0.11.15

As you can see, werkzeug was updated as well.