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 (including any command
line options) 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 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 14
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 happily manipulate files in the directory of a
   submodule that has just been removed in the superproject (but is of
   course still present in the work tree due to the way submodules are
   currently handled). This becomes dangerous when files in the submodule
   directory are overwritten by files from the new superproject commit, as
   any modifications to the submodule files will be lost) and is expected
   to also destroy history in the - admittedly unlikely case - the new
   commit adds a file named ".git" to 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>
bisect: add t6039 for submodule updates

Test that the bisect command updates the work tree as expected. To make
that work with the new submodule test framework a git_bisect helper
function is added. This adds a commit after the one given to be switched
to and makes that one the bad commit. The starting point is then given to
bisect as the good commit which makes bisect change the work tree to the
commit in between, which is the commit given.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
rebase: add t3426 for submodule updates

Test that the rebase command updates the work tree as expected for changes
which don't result in conflicts.

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

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.

Add the KNOWN_FAILURE_NOFF_MERGE_DOESNT_CREATE_EMPTY_SUBMODULE_DIR switch
to expect the known failure that --no-ff merges do not create the empty
submodule directory.

Add the KNOWN_FAILURE_NOFF_MERGE_ATTEMPTS_TO_MERGE_REMOVED_SUBMODULE_FILES
switch to expect the known failure that --no-ff merges attempt to merge
the new files in the former submodule directory with those of the removed
submodule.

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

TODO: check why the two failures from merge do not happen here

Test that the pull 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.

Set KNOWN_FAILURE_NOFF_MERGE_DOESNT_CREATE_EMPTY_SUBMODULE_DIR to
document that pull has the same --no-ff known failure merge has.

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).

Set KNOWN_FAILURE_NOFF_MERGE_ATTEMPTS_TO_MERGE_REMOVED_SUBMODULE_FILES
and KNOWN_FAILURE_NOFF_MERGE_DOESNT_CREATE_EMPTY_SUBMODULE_DIR to
document that pull has the same --no-ff known failure merge has.

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>
am: add t4255 for submodule updates

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

Add the KNOWN_FAILURE_NOFF_MERGE_ATTEMPTS_TO_MERGE_REMOVED_SUBMODULE_FILES
switch to expect the known failure that --no-ff merges attempt to merge
the new files in the former submodule directory with those of the removed
submodule.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
!!! WIP !!! stash: add t3906 for submodule updates

TODO: properly set up work tree in test helper functions

Test that the stash apply command updates the work tree as expected for
changes which don't result in conflicts no matter if submodule changes are
staged or not.

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