rebase -i: more graceful handling of invalid commands

rebase -i: more graceful handling of invalid commands

Currently, when there is an invalid command, the rest of the line is
still treated as if the command had been valid, i.e. rebase -i attempts
to produce a patch, using the next argument as a SHA1 name. If there is
no next argument or an invalid one, very confusing error messages
appear (the line was '.'; path to git-rebase-todo substituted):

Unknown command: .
fatal: ambiguous argument 'Please fix this in the file $somefile.':
unknown revision or path not in the working tree.
Use '--' to separate paths from revisions
fatal: Not a valid object name Please fix this in the file $somefile.
fatal: bad revision 'Please fix this in the file $somefile.'

Instead, verify the validity of the remaining line and error out earlier
if necessary.

Signed-off-by: Jan Krüger <jk@jk.gs>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>