Only change home files ownership for --rebuild mode

Changing ownership of the entire $HOME directory tree is sometimes an
expensive task and a no-op anyway because there's typically no reason
for files with the wrong ownership to appear. Let's stop doing it to
make the subsequent mock --chroot calls faster and more predictable.

In other words, if users want to shoot themselves in the foot and give
the files in '/builddir' away (e.g., using "privileged" --chroot calls),
Mock will no longer fix the ownership problems.  We only continue to fix
the permissions for the --rebuild mode, which shouldn't be expensive
(because there are pre-build cleanups) and seems important enough to
risk changing.

Suggested by @kdudka in #1257.

mock/py/mockbuild/buildroot.py

@@ -348,7 +348,8 @@ def _init(self, prebuild):
 
         # Change owner of homdir tree if the root of it not owned
         # by the current user
-        self.chown_home_dir()
+        if prebuild:
+            self.chown_home_dir()
 
         # mark the buildroot as initialized
         file_util.touch(self.make_chroot_path('.initialized'))