wt-status.*: better advices for git status added

wt-status.*: better advices for git status added

This patch provides new informative help messages in the display of
'git status' (at the top) during conflicts, rebase, am, bisect or
cherry-pick process.

The new messages are not shown when using options such as -s or
--porcelain. The messages about the current situation of the user are
always displayed but the advices on what the user needs to do in order
to resume a rebase/bisect/am/commit after resolving conflicts can be
hidden by setting advice.statushints to 'false' in the config file.

Thus, information about the updated advice.statushints key are added
in Documentation/config.txt.

Also, the test t7060-wt-status.sh is now working with the new help
messages. Tests about suggestions of "git rm" are also added.

Signed-off-by: Lucien Kong <Lucien.Kong@ensimag.imag.fr>
Signed-off-by: Valentin Duperray <Valentin.Duperray@ensimag.imag.fr>
Signed-off-by: Franck Jonas <Franck.Jonas@ensimag.imag.fr>
Signed-off-by: Thomas Nguy <Thomas.Nguy@ensimag.imag.fr>
Signed-off-by: Huynh Khoi Nguyen Nguyen <Huynh-Khoi-Nguyen.Nguyen@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>