zsh: add gflat script
zsh: add gflat script

This makes easier the tedious act of running "git rebase -i ..."
followed by using "fixup" for all child commits up to the oldest commit
in the rebase buffer.

Before, our workflow was, for example "grh 10" followed by editing the
buffer by hand, but now it's just "gflat 10". This command also takes a
non-numerical (commitish) argument for more flexibility.

listx/syscfg,listx/syscfg,listx/syscfg,listx/syscfg,listx/syscfg