core-tutorial: Catch up with current Git

core-tutorial: Catch up with current Git

No longer talk about Cogito since it's deprecated.  Some scripts (such as
git-reset or git-branch) have undergone builtinification so adjust the text
to reflect this.

Fix a typo in the description of git-show-branch (merges are indicated by a
`-', not by a `.').

git-pull/git-push do not seem to use the dumb git-ssh-fetch/git-ssh-upload
(the text was probably missing a word).

Adjust a link that wasn't rendered properly because it was wrapped.

Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>