Feature: use git branches/tags as versions (#31200)

Building on #24639, this allows versions to be prefixed by `git.`. If a version begins `git.`, it is treated as a git ref, and handled as git commits are starting in the referenced PR.

An exception is made for versions that are `git.develop`, `git.main`, `git.master`, `git.head`, or `git.trunk`. Those are assumed to be greater than all other versions, as those prefixed strings are in other contexts.

var/spack/repos/builtin/packages/libcatalyst/package.py

@@ -16,7 +16,7 @@ class Libcatalyst(CMakePackage):
     maintainers = ['mathstuf']
 
     # master as of 2021-05-12
-    version('8456ccd6015142b5a7705f79471361d4f5644fa7', sha256='5a01f12b271d9d9e9b89f31d45a5f4b8426904483639d38754893adfd3547bab')
+    version('2021-05-12', sha256='5a01f12b271d9d9e9b89f31d45a5f4b8426904483639d38754893adfd3547bab')
 
     variant('mpi', default=False, description='Enable MPI support')
     variant('python3', default=False, description='Enable Python3 support')