Allow "spack install foo@git-hash" without terminal prompt (#29827)

Do not prompt user with checksum warning when using git commit hashes
as versions. Spack was incorrectly reporting this as a potential
problem: it would display a prompt asking the user whether they
want to proceed if Spack was running in a terminal, or it would
terminate the running instance of Spack if running as part of a
script.

lib/spack/spack/package.py

@@ -1474,7 +1474,8 @@ def do_fetch(self, mirror_only=False):
 
         checksum = spack.config.get('config:checksum')
         fetch = self.stage.managed_by_spack
-        if checksum and fetch and self.version not in self.versions:
+        if checksum and fetch and (self.version not in self.versions) \
+                and (not self.version.is_commit):
             tty.warn("There is no checksum on file to fetch %s safely." %
                      self.spec.cformat('{name}{@version}'))