patcht/t7700-repack.sh: fix test breakages with `GIT_TEST_MULTI_PACK_INDEX=1`

> There are a handful of related test breakages which are found when
> running t/t7700-repack.sh with GIT_TEST_MULTI_PACK_INDEX set to "1" in
> your environment.
>
> Both test failures are the result of something like:
>
>     git repack --write-midx --write-bitmap-index [...] &&
>
>     test_path_is_file $midx &&
>     test_path_is_file $midx-$(midx_checksum $objdir).bitmap
>
> , where we repack instructing Git to write a new MIDX and corresponding
> MIDX bitamp.
>
> The error occurs when GIT_TEST_MULTI_PACK_INDEX=1 is found in the
> enviornment. This causes Git to write out a second MIDX (after
> processing the builtin's `--write-midx` argument) which is identical to
> the first, but does not request a bitmap (since we did not set the
> GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP variable in the environment).

> diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
> index 94f9f4a1da..127efe99f8 100755
> --- a/t/t7700-repack.sh
> +++ b/t/t7700-repack.sh
> @@ -629,6 +629,7 @@ test_expect_success '--write-midx with preferred bitmap tips' '
>  		git log --format="create refs/tags/%s/%s %H" HEAD >refs &&
>  		git update-ref --stdin <refs &&
>  
> +		GIT_TEST_MULTI_PACK_INDEX=0 \
>  		git repack --write-midx --write-bitmap-index &&
>  		test_path_is_file $midx &&
>  		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&

> Taylor Blau <me@ttaylorr.com> writes:
>
> > There are a handful of related test breakages which are found when
> > running t/t7700-repack.sh with GIT_TEST_MULTI_PACK_INDEX set to "1" in
> > your environment.
> >
> > Both test failures are the result of something like:
> >
> >     git repack --write-midx --write-bitmap-index [...] &&
> >
> >     test_path_is_file $midx &&
> >     test_path_is_file $midx-$(midx_checksum $objdir).bitmap
> >
> > , where we repack instructing Git to write a new MIDX and corresponding
> > MIDX bitamp.
> >
> > The error occurs when GIT_TEST_MULTI_PACK_INDEX=1 is found in the
> > enviornment. This causes Git to write out a second MIDX (after
> > processing the builtin's `--write-midx` argument) which is identical to
> > the first, but does not request a bitmap (since we did not set the
> > GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP variable in the environment).
>
> Doesn't it sound more like a bug, though?  If a command line option
> requests something, should we still be honoring a contradicting
> instruction given by environment variable(s)?
>
> But anyway.

> > diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
> > index 94f9f4a1da..127efe99f8 100755
> > --- a/t/t7700-repack.sh
> > +++ b/t/t7700-repack.sh
> > @@ -629,6 +629,7 @@ test_expect_success '--write-midx with preferred bitmap tips' '
> >  		git log --format="create refs/tags/%s/%s %H" HEAD >refs &&
> >  		git update-ref --stdin <refs &&
> >
> > +		GIT_TEST_MULTI_PACK_INDEX=0 \
> >  		git repack --write-midx --write-bitmap-index &&
> >  		test_path_is_file $midx &&
> >  		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
>
> Is it a viable alternative approach to skip this check (and the
> other one) when GIT_TEST_MULTI_PACK_INDEX is set (i.e., lazy
> prereq).  It will give us a better documentation value, e.g.,
>
> 	test_lazy_prereq FORCED_MIDX '
>                 # Features that are broken when GIT_TEST_* forces it
>                 # to enable are protexted with this prerequisite.
> 		test "$GIT_TEST_MULTI_PACK_INDEX" = 1;
> 	'
>
> 	test_expect_success !FORCED_MIDX '--write-midx with ...' '
> 		...
> 	'
>
> With a single comment, we can annotate any future tests that relies
> on features working correctly even with GIT_TEST_MULTI_PACK_INDEX.

> I would argue that GIT_TEST_MULTI_PACK_INDEX should be on the list of
> GIT_TEST_-variables to get rid of as it has served its purpose.
>
> Like I said, I'd like to get rid of this (and many other)
> GIT_TEST-related variables, but that is a larger effort than this single
> patch.

> Taylor Blau <me@ttaylorr.com> writes:
>
> > I would argue that GIT_TEST_MULTI_PACK_INDEX should be on the list of
> > GIT_TEST_-variables to get rid of as it has served its purpose.
> >
> > Like I said, I'd like to get rid of this (and many other)
> > GIT_TEST-related variables, but that is a larger effort than this single
> > patch.
>
> Yup, that sounds like a good longer-term goals.  While it does not
> smell like it is consistent with that goal to add more instances of
> use of it to the test, it may inevitably be a "few steps backward in
> preparation to jump big forward", perhaps.

> I have generally considered the `--write-midx` and
> `GIT_TEST_MULTI_PACK_INDEX` options to be orthogonal to each other. The
> latter is a developer-oriented option that forces Git to write a MIDX
> post-repack regardless of the command-line option.
> 
> It predates the `--write-midx` option by a number of years, and IIUC was
> introduced to enhance test coverage while the MIDX was being originally
> developed.
> 
> I would argue that GIT_TEST_MULTI_PACK_INDEX should be on the list of
> GIT_TEST_-variables to get rid of as it has served its purpose.