add 'git credential' plumbing command

The credential API is in C, and not available to scripting languages.
Expose the functionalities of the API by wrapping them into a new
plumbing command "git credentials".

In other words, replace the internal "test-credential" by an official Git
command.

Most documentation writen by: Jeff King <peff@peff.net>
Signed-off-by: Pavel Volek <Pavel.Volek@ensimag.imag.fr>
Signed-off-by: Kim Thuat Nguyen <Kim-Thuat.Nguyen@ensimag.imag.fr>
Signed-off-by: Javier Roucher Iglesias <Javier.Roucher-Iglesias@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-credential.sh

@@ -4,10 +4,20 @@
 # stdout and stderr should be provided on stdin,
 # separated by "--".
 check() {
+	credential_opts=
+	credential_cmd=$1
+	shift
+	for arg in "$@"; do
+		credential_opts="$credential_opts -c credential.helper='$arg'"
+	done
 	read_chunk >stdin &&
 	read_chunk >expect-stdout &&
 	read_chunk >expect-stderr &&
-	test-credential "$@" <stdin >stdout 2>stderr &&
+	if ! eval "git $credential_opts credential $credential_cmd <stdin >stdout 2>stderr"; then
+		echo "git credential failed with code $?" &&
+		cat stderr &&
+		false
+	fi &&
 	test_cmp expect-stdout stdout &&
 	test_cmp expect-stderr stderr
 }
@@ -41,7 +51,7 @@ reject() {
 		echo protocol=$2
 		echo host=$3
 		echo username=$4
-	) | test-credential reject $1
+	) | git -c credential.helper=$1 credential reject
 }
 
 helper_test() {