test-lib functions: add --printf option to test_commit

Add a --printf option to test_commit to allow writing to the file with
"printf" instead of "echo".

This is useful for writing "\n", "\0" etc., in particular in
combination with the --append option added in 3373518cc8 (test-lib
functions: add an --append option to test_commit, 2021-01-12).

I'm converting a few tests to use the new option rather than a manual
printf/add/commit combination to demonstrate its usefulness. While I'm
at it use "test_create_repo" where appropriate, and give the
first/second commit a meaningful/more conventional log message in
cases where no test cared about that message.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -173,6 +173,12 @@ debug () {
 #	Do not call test_tick before making a commit
 #   --append
 #	Use ">>" instead of ">" when writing "<contents>" to "<file>"
+#   --printf
+#       Use "printf" instead of "echo" when writing "<contents>" to
+#       "<file>", use this to write escape sequences such as "\0", a
+#       trailing "\n" won't be added automatically. This option
+#       supports nothing but the FORMAT of printf(1), i.e. no custom
+#       ARGUMENT(s).
 #   --signoff
 #	Invoke "git commit" with --signoff
 #   --author <author>
@@ -191,6 +197,7 @@ debug () {
 
 test_commit () {
 	notick= &&
+	echo=echo &&
 	append= &&
 	author= &&
 	signoff= &&
@@ -202,6 +209,9 @@ test_commit () {
 		--notick)
 			notick=yes
 			;;
+		--printf)
+			echo=printf
+			;;
 		--append)
 			append=yes
 			;;
@@ -238,9 +248,9 @@ test_commit () {
 	file=${2:-"$1.t"} &&
 	if test -n "$append"
 	then
-		echo "${3-$1}" >>"$indir$file"
+		$echo "${3-$1}" >>"$indir$file"
 	else
-		echo "${3-$1}" >"$indir$file"
+		$echo "${3-$1}" >"$indir$file"
 	fi &&
 	git ${indir:+ -C "$indir"} add "$file" &&
 	if test -z "$notick"