t1010: fix unnoticed failure on Windows

On Microsoft Windows, a directory name should never end with a period.
Quoting from Microsoft documentation[1]:

    Do not end a file or directory name with a space or a period.
    Although the underlying file system may support such names, the
    Windows shell and user interface does not.

Naming a directory with a trailing period is indeed perilous:

    % git init foo
    % cd foo
    % mkdir a.
    % git status
    warning: could not open directory 'a./': No such file or directory

The t1010 "setup" test:

    for d in a a. a0
    do
        mkdir "$d" && echo "$d/one" >"$d/one" &&
        git add "$d"
    done &&

runs afoul of this Windows limitation, as can be observed when running
the test verbosely:

    error: open("a./one"): No such file or directory
    error: unable to index file 'a./one'
    fatal: adding files failed

The reason this problem has gone unnoticed for so long is twofold.
First, the failed `git add` is swallowed silently because the loop is
not terminated explicitly by `|| return 1` to signal the failure.
Second, none of the tests in this script care about the literal
directory names ("a", "a.", "a0") or the specific number of tree
entries. They care instead about the order of entries in the tree, and
that the tree synthesized in the index and created by `git write-tree`
matches the tree created by the output of `git ls-tree` fed into `git
mktree`, thus the absence of "a./one" has no impact on the tests.

Skipping these tests on Windows by, for instance, checking the
FUNNYNAMES predicate would avoid the problem, however, the funny-looking
name is not what is being tested here. Rather, the tests are about
checking that `git mktree` produces stable results for various input
conditions, such as when the input order is not consistent or when an
object is missing.

Therefore, resolve the problem simply by using a directory name which is
legal on Windows and sorts the same as "a.". While at it, add the
missing `|| return 1` to the loop body in order to catch this sort of
problem in the future.

[1]: https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1010-mktree.sh

@@ -6,10 +6,10 @@ TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success setup '
-	for d in a a. a0
+	for d in a a- a0
 	do
 		mkdir "$d" && echo "$d/one" >"$d/one" &&
-		git add "$d"
+		git add "$d" || return 1
 	done &&
 	echo zero >one &&
 	git update-index --add --info-only one &&