magit.el: remove `git-commit-commit-function' declaration

Obsolete since magit/git-modes@e0763fdc.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-assert-emacsclient': s/tramp-file-name-p/tramp-tramp-file-p/

Running `magit-interactive-rebase' in a repo over TRAMP resulted in:

  $ git --no-pager rebase -i foo^
  fatal: Terminal is dumb, but EDITOR unset
  Could not execute editor
  git exited abnormally with code 1.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-interactive-rebase': pass more specific ACTION to `magit-assert-emacsclient'

We only need emacsclient(1) when rebasing *interactively*, so
don't needlessly discourage the user from using ordinary rebase.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #810 from praet/sane-client-fixes

sane-client fixes