config.txt: document include, includeIf

config.txt: document include, includeIf

Git config's tab completion does not yet know about the "include"
and "includeIf" sections, nor the related "path" variable.

Add a description for these two sections in
'Documentation/config/includeif.txt', which points to git-config's
documentation, specifically the "Includes" and "Conditional Includes"
subsections.

As a side effect, tab completion can successfully complete the
'include', 'includeIf', and 'include.add' expressions.
This effect is tested by two new ad-hoc tests.
Variable completion only works for "include" for now.

Credit for the ideas behind this patch goes to
Ævar Arnfjörð Bjarmason.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Manuel Boni <ziosombrero@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>