Merge branch 'rt/format-zero-length-fix'

Merge branch 'rt/format-zero-length-fix'

Recently we inadvertently added a few instances of using 0-width
format string to functions that we mark as printf-like without any
developers noticing.  The root cause was that the compiler warning
that is triggered by this is almost always useless and we disabled
the warning in our developer builds, but not for general public.
The new instances have been corrected, and the warning has been
resurrected in the developer builds.

* rt/format-zero-length-fix:
  config.mak.dev: re-enable -Wformat-zero-length
  rebase-interactive.c: silence format-zero-length warnings