bisect: fix "git bisect skip <commit>" and add tests cases

bisect: fix "git bisect skip <commit>" and add tests cases

The patch that allows "git bisect skip" to be passed a range of
commits using the "<commit1>..<commit2>" notation is flawed because
it introduces a regression when it was passed a simple rev or commit.

"git bisect skip <commit>" doesn't work any more, because <commit>
is quoted but not properly unquoted.

This patch fixes that and add tests cases to better check when it is
passed commits and range of commits.

While at it, this patch also properly quotes the non range arguments
using the "sq" function.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>