doc lint: lint and fix missing "GIT" end sections

doc lint: lint and fix missing "GIT" end sections

Lint for and fix the three manual pages that were missing the standard
"Part of the linkgit:git[1] suite" end section.

We only do this for the man[157] section documents (we don't have
anything outside those sections), not files to be included,
howto *.txt files etc.

We could also add this to the existing (and then renamed)
lint-gitlink.perl, but I'm not doing that here.

Obviously all of that fits in one script, but I think for something
like this that's a one-off script with global variables it's much
harder to follow when a large part of your script is some if/else or
keeping/resetting of state simply to work around the script doing two
things instead of one.

Especially because in this case this script wants to process the file
as one big string, but lint-gitlink.perl wants to look at it one line
at a time. We could also consolidate this whole thing and
t/check-non-portable-shell.pl, but that one likes to join lines as
part of its shell parsing.

So let's just add another script, whole scaffolding is basically:

    use strict;
    use warnings;
    sub report { ... }
    my $code = 0;
    while (<>) { ... }
    exit $code;

We'd spend more lines effort trying to consolidate them than just
copying that around.

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