Make negative look-ahead only match exact lines

Fixes subtle problems like:

-GRUB_CMDLINE_LINUX_DEFAULT='console=ttyS0,115200n1 elevator=deadline'=ttyS0,115200n1 elevator=deadline'=ttyS0,115200n1 elevator=deadline'=ttyS0,115200n1 elevator=deadline'
-GRUB_CMDLINE_LINUX='console=tty0'=tty0'=tty0'=tty0'
+GRUB_CMDLINE_LINUX_DEFAULT='console=ttyS0,115200n1 elevator=deadline'
+GRUB_CMDLINE_LINUX='console=tty0'