Address issue #4331, "working copy permissions change on commit for files
with keywords" by preventing the accidental removal of existing permission
bits set on files which undergo translation.

This is not a perfect answer to the proposition given in the issue that
"Committing a file should not change its permissions in the WC."
Committed files may still get a new owner/group/other write bit set if
the umask allows this and the file's properties indicate that the file must
be set read-write (see svn_wc__sync_flags_with_props() which might tweak
permission bits of the translated file). But I think this is acceptable.
I'm not sure if never changing the permission bits of committed files is
a realistic requirement. Changing this would require additional effort
and might break other scenarios.

* subversion/libsvn_subr/subst.c
  (svn_subst_copy_and_translate4): Copy file permissions from the source file
   to the temporary file which stores the translated result. The temporary
   file might have stricter permissions than the source, due to the way
   our APIs for creating temporary files work. Removing permission bits from
   files during translation is an unwanted side effect and should be avoided.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1452780 13f79535-47bb-0310-9956-ffa450edef68