Fix extended short SHA1 name completion

get_sha1() would not do sha1 completion of short SHA1's when they were
part of a more complex expression.  So doing

git-rev-parse 727132834e6be48a93c1bd6458a29d474ce7d5d5^

would work, and return 87c6aeb4efdd4355918d127a91bd0adc5a02f8ff. But using
the shorthand version

git-rev-list 72713^

wouldn't work.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>