[PATCH v3 2/4] test-lib: correct commentary on TEST_DIRECTORY overriding

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Correct a misleading comment added by me in 62f539043c7 (test-lib:
Allow overriding of TEST_DIRECTORY, 2010-08-19).

Between that comment and the later addition of
85176d72513 (test-lib.sh: convert $TEST_DIRECTORY to an absolute path,
2013-11-17) the comments were on the wrong arms of the "if". I.e. the
"allow tests to override this" was on the "test -z" arm.

But more importantly this could be read allowing the "$TEST_DIRECTORY"
to be some path outside of t/. As explained in the updated comment
that's impossible, rather it was meant for *tests* that ran outside of
t/, i.e. the "t0000-basic.sh" tests that use "lib-subtest.sh".

Those tests have a different working directory, but they set the
"TEST_DIRECTORY" to the same path for bootstrapping. The comments now
reflect that, and further comment on why we have a hard dependency on
this.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 t/test-lib.sh | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 55f263a02d3..77c3fabd041 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -19,13 +19,20 @@
 # t/ subdirectory and are run in 'trash directory' subdirectory.
 if test -z "$TEST_DIRECTORY"
 then
-	# We allow tests to override this, in case they want to run tests
-	# outside of t/, e.g. for running tests on the test library
-	# itself.
-	TEST_DIRECTORY=$(pwd)
-else
 	# ensure that TEST_DIRECTORY is an absolute path so that it
 	# is valid even if the current working directory is changed
+	TEST_DIRECTORY=$(pwd)
+else
+	# The TEST_DIRECTORY will always be the path to the "t"
+	# directory in the git.git checkout. This is overridden by
+	# e.g. t/lib-subtest.sh, but only because its $(pwd) is
+	# different. Those tests still set "$TEST_DIRECTORY" to the
+	# same path.
+	#
+	# See use of "$GIT_BUILD_DIR" and "$TEST_DIRECTORY" below for
+	# hard assumptions about "$GIT_BUILD_DIR/t" existing and being
+	# the "$TEST_DIRECTORY", and e.g. "$TEST_DIRECTORY/helper"
+	# needing to exist.
 	TEST_DIRECTORY=$(cd "$TEST_DIRECTORY" && pwd) || exit 1
 fi
 if test -z "$TEST_OUTPUT_DIRECTORY"
-- 
2.35.1.1132.ga1fe46f8690