Let git-help prefer man-pages installed with this version of git

Let git-help prefer man-pages installed with this version of git

Prepend $(prefix)/share/man to the MANPATH environment variable before
invoking 'man' from help.c:show_man_page().  There may be other git
documentation in the user's MANPATH but the user is asking a specific
instance of git about its own documentation, so we'd better show the
documentation for _that_ instance of git.

Signed-off-by: Sergei Organov <osv@javad.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>