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>
diff --git a/git.c b/git.c
index 4da3db5..8788b32 100644
--- a/git.c
+++ b/git.c
@@ -351,6 +351,7 @@
 		{ "commit-tree", cmd_commit_tree, RUN_SETUP },
 		{ "config", cmd_config, RUN_SETUP_GENTLY },
 		{ "count-objects", cmd_count_objects, RUN_SETUP },
+		{ "credential", cmd_credential, RUN_SETUP_GENTLY },
 		{ "describe", cmd_describe, RUN_SETUP },
 		{ "diff", cmd_diff },
 		{ "diff-files", cmd_diff_files, RUN_SETUP | NEED_WORK_TREE },