t/lib-terminal: make TTY a lazy prerequisite

From: Jeff King <peff@peff.net>

When lib-terminal.sh is sourced by a test script, we
immediately set up the TTY prerequisite. We do so inside a
test_expect_success, because that nicely isolates any
generated output.

However, this early test can interfere with a script that
later wants to skip all tests (e.g., t5541 then goes on to
set up the httpd server, and wants to skip_all if that
fails). TAP output doesn't let us skip everything after we
have already run at least one test.

We could fix this by reordering the inclusion of
lib-terminal.sh in t5541 to go after the httpd setup.  That
solves this case, but we might eventually hit a case with
circular dependencies, where either lib-*.sh include might
want to skip_all after the other has run a test.  So
instead, let's just remove the ordering constraint entirely
by doing the setup inside a test_lazy_prereq construct,
rather than in a regular test.  We never cared about the
test outcome anyway (it was written to always succeed).

Note that in addition to setting up the prerequisite, the
current test also defines test_terminal. Since we can't
affect the environment from a lazy_prereq, we have to hoist
that out. We previously depended on it _not_ being defined
when the TTY prereq isn't set as a way to ensure that tests
properly declare their dependency on TTY. However, we still
cover the case (see the in-code comment for details).

Reported-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Jeff King <peff@peff.net>
test-lib: add test_dir_is_empty()

For the upcoming submodule test framework we often need to assert that an
empty directory exists in the work tree. Add the test_dir_is_empty()
function which asserts that the given argument is an empty directory.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Submodules: Add the lib-submodule-update.sh test library

Add this test library to simplify covering all combinations of submodule
update scenarios without having to add those to a test of each work tree
manipulating command over and over again.

The functions test_submodule_switch() and test_submodule_forced_switch()
are intended to be called from a test script with a single argument. This
argument is either a work tree manipulating command or a function (when
more than a single git command is needed to switch work trees from the
current HEAD to another commit). This command (or function) is passed a
target branch as argument. The two new functions check that each submodule
transition is handled as expected, which currently means that submodule
work trees are not affected by current git commands until "git submodule
update" is called. The "forced" variant is for commands using their '-f'
or '--hard' option and expects them to overwrite local modifications as a
result. Each of these two functions contains 17 tests_expect_* calls.

Calling one of these test functions the first time creates a repository
named "submodule_update_repo". At first it contains two files, then a
single submodule is added in another commit followed by commits covering
all relevant submodule modifications. This repository is newly cloned into
the "submodule_update" for each test_expect_* to avoid interference
between different parts of the test functions (some to-be-tested commands
also manipulate refs along with the work tree, e.g. "git reset").

Follow-up commits will then call these two test functions for all work
tree manipulating commands (with a combination of all their options
relevant to what they do with the work tree) making sure they work as
expected. Later this test library will be extended to cover merges
resulting in conflicts too. Also it is intended to be easily extendable
for the recursive update functionality, where even more combinations of
submodule modifications have to be tested for.

This version documents two bugs in current Git with expected failures:

*) When a submodule is replaced with a tracked file of the same name the
   submodule work tree including any local modifications (and even the
   whole history if it uses a .git directory instead of a gitfile!) is
   simply removed.

*) Forced work tree updates manipulate the work tree of a submodule that
   has been removed in the superproject's work tree (which isn't removed
   yet due to the way submodules are handled). This is dangerous if files
   in the submodule are overwritten by files from the new commit, as any
   modifications will be lost) and is expected to also destroy history in
   the - admittedly unlikely case - the new commit tracks a file named
   ".git" in the submodule directory.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
checkout: call the new submodule update test framework

Test that the checkout command updates the work tree as expected with or
without the '-f' flag.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
apply: add t4137 for submodule updates

Test that the apply command updates the work tree as expected for the
'--index' and the '--3way' options (for submodule changes which don't
result in conflicts).

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
read-tree: add t1013 for submodule updates

Test that the read-tree command updates the work tree as expected for
changes which don't result in conflicts with the '-m' and '--reset' flag.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
reset: add t7112 for submodule updates

Test that the reset command updates the work tree as expected for changes
with '--keep', '--merge' (for changes which don't result in conflicts) and
'--hard'.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! merge: add t7613 for submodule updates

TODO: fix or expect failing tests

Test that the merge command updates the work tree as expected (for
submodule changes which don't result in conflicts) when used without
arguments or with the '--ff', '--ff-only' and '--no-ff' flag.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! pull: add t5572 for submodule updates

TODO: fix or expect failing tests

Test that the merge command updates the work tree as expected (for
submodule changes which don't result in conflicts) when used without
arguments or with the '--ff', '--ff-only' and '--no-ff' flag each. Add
helper functions to reset the work tree to the commit before the one to
update the work tree to so that pull has something to do.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! cherry-pick: add t3512 for submodule updates

TODO: fix or expect failing tests

Test that the cherry-pick command updates the work tree as expected (for
submodule changes which don't result in conflicts).

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! checkout-index: add t2025 for submodule updates

TODO: fix or expect failing tests

Test that the checkout-index command updates the work tree as expected
(for submodule changes which don't result in conflicts) with or without
the '-f' flag.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! reminder of what is still left to do ...