git-stash: improve synopsis in help and manual page

git-stash: improve synopsis in help and manual page

"git stash -h" showed some incomplete and ugly usage information.
For example, the useful "--keep-index" option for "save" or the "--index"
option for  "apply" were not shown. Also in the documentation synopsis they
were not shown, so that there is no incentive to scroll down and even see
that such options exist.

This patch improves the git-stash synopsis in the documentation by
mentioning that further options to the stash commands and then copies
this synopsis to the usage information string of git-stash.sh.

For the latter, the dashless git command string has to be inserted on the
second and the following usage lines. The code of this is taken from
git-sh-setup so that all lines will show the command string.

Note that the "create" command is not advertised at all now, because
it was not mentioned in git-stash.txt.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>