subtree: t7900: comment subtree_test_create_repo

It's unclear what the purpose of t7900-subtree.sh's
`subtree_test_create_repo` helper function is.  It wraps test-lib.sh's,
`test_create_repo` but follows that up by setting log.date=relative.  Why
does it set log.date=relative?

My first guess was that at one point the tests required that, but no
longer do, and that the function is now vestigial.  I even wrote a patch
to get rid of it and was moments away from `git send-email`ing it.

However, by chance when looking for something else in the history, I
discovered the true reason, from e7aac44 (contrib/subtree: ignore
log.date configuration, 2015-07-21).  It's testing that setting
log.date=relative doesn't break `git subtree`, as at one point in the past
that did break `git subtree`.

So, add a comment about this, to avoid future such confusion.

And while at it, go ahead and (1) touch up the function to avoid a
pointless subshell and (2) update the one test that didn't use it.

Signed-off-by: Luke Shumaker <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: LukeShu

contrib/subtree/t/t7900-subtree.sh

@@ -13,12 +13,14 @@ TEST_DIRECTORY=$(pwd)/../../../t
 export TEST_DIRECTORY
 . "$TEST_DIRECTORY"/test-lib.sh
 
+# Use our own wrapper around test-lib.sh's test_create_repo, in order
+# to set log.date=relative.  `git subtree` parses the output of `git
+# log`, and so it must be careful to not be affected by settings that
+# change the `git log` output.  We test this by setting
+# log.date=relative for every repo in the tests.
 subtree_test_create_repo () {
 	test_create_repo "$1" &&
-	(
-		cd "$1" &&
-		git config log.date relative
-	)
+	git -C "$1" config log.date relative
 }
 
 create () {
@@ -242,8 +244,8 @@ test_expect_success 'merge the added subproj again, should do nothing' '
 '
 
 test_expect_success 'merge new subproj history into subdir/ with a slash appended to the argument of --prefix' '
-	test_create_repo "$test_count" &&
-	test_create_repo "$test_count/subproj" &&
+	subtree_test_create_repo "$test_count" &&
+	subtree_test_create_repo "$test_count/subproj" &&
 	test_create_commit "$test_count" main1 &&
 	test_create_commit "$test_count/subproj" sub1 &&
 	(