bin/git-land: use COMMIT_EDITMSG filename
bin/git-land: use COMMIT_EDITMSG filename

Some editors (like vim) will interpret a file as a git commit message
if its filename is `COMMIT_EDITMSG`. It's helpful for automatically
wrapping lines in commit messages.

https://github.com/vim/vim/blob/42ebd066422d73cdb7bda6a1dc828a3dd022dec8/runtime/filetype.vim#L782

Closes chain/chainprv#1133.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr



chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain