I have some package installed before 075c3e0. When I update Spack to a newer commit, all spack install commands try to reinstall all of my already installed packages. When I explicitly tell it to use a particular hash for the dependency, it is completely ignored and still rebuilds everything.

Steps to reproduce the issue

$ git clone https://github.com/spack/spack
$ git checkout 075c3e0d9247e9c1716ac44e7052366ac59ff110~1
$ spack install python
$ git checkout 075c3e0d9247e9c1716ac44e7052366ac59ff110
$ spack spec -I python
$ spack spec -I /hashofpython

Information on your system

• Spack: 0.15.4-1279-eabfdba08f
• Python: 3.8.5
• Platform: darwin-catalina-ivybridge

Additional information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output

@scottwittenburg I believe this bug was introduced in #18359, can you take a look?