builtin/stash.c: let parse-options parse subcommands

builtin/stash.c: let parse-options parse subcommands

'git stash' parses its subcommands with a long list of if-else if
statements.  parse-options has just learned to parse subcommands, so
let's use that facility instead, with the benefits of shorter code,
and listing subcommands for Bash completion.

Note that the push_stash() function implementing the 'push' subcommand
accepts an extra flag parameter to indicate whether push was assumed,
so add a wrapper function with the standard subcommand function
signature.

Note also that this change "hides" the '-h' option in 'git stash push
-h' from the parse_option() call in cmd_stash(), as it comes after the
subcommand.  Consequently, from now on it will emit the usage of the
'push' subcommand instead of the usage of 'git stash'.  We had a
failing test for this case, which can now be flipped to expect
success.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>