completion: remove 'help' duplicate from porcelain commands

completion: remove 'help' duplicate from porcelain commands

The list of all git commands is computed from the output of 'git help
-a', which already includes 'help', so there is no need to explicitly
add it once more when computing the list of porcelain commands.

Note that 'help' wasn't actually offered twice because of this,
because Bash filters duplicates from possible completion words.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>