Revert "Build-in "git-shell""

This reverts commit daa0cc9a92.
It was a stupid idea to do this; when run as a log-in shell,
it is spawned with argv[0] set to "-git-shell", so the usual
name-based dispatch would not work to begin with.

shell.c

@@ -0,0 +1,89 @@
+#include "cache.h"
+#include "quote.h"
+#include "exec_cmd.h"
+#include "strbuf.h"
+
+static int do_generic_cmd(const char *me, char *arg)
+{
+	const char *my_argv[4];
+
+	setup_path();
+	if (!arg || !(arg = sq_dequote(arg)))
+		die("bad argument");
+	if (prefixcmp(me, "git-"))
+		die("bad command");
+
+	my_argv[0] = me + 4;
+	my_argv[1] = arg;
+	my_argv[2] = NULL;
+
+	return execv_git_cmd(my_argv);
+}
+
+static int do_cvs_cmd(const char *me, char *arg)
+{
+	const char *cvsserver_argv[3] = {
+		"cvsserver", "server", NULL
+	};
+
+	if (!arg || strcmp(arg, "server"))
+		die("git-cvsserver only handles server: %s", arg);
+
+	setup_path();
+	return execv_git_cmd(cvsserver_argv);
+}
+
+
+static struct commands {
+	const char *name;
+	int (*exec)(const char *me, char *arg);
+} cmd_list[] = {
+	{ "git-receive-pack", do_generic_cmd },
+	{ "git-upload-pack", do_generic_cmd },
+	{ "cvs", do_cvs_cmd },
+	{ NULL },
+};
+
+int main(int argc, char **argv)
+{
+	char *prog;
+	struct commands *cmd;
+
+	/*
+	 * Special hack to pretend to be a CVS server
+	 */
+	if (argc == 2 && !strcmp(argv[1], "cvs server"))
+		argv--;
+
+	/*
+	 * We do not accept anything but "-c" followed by "cmd arg",
+	 * where "cmd" is a very limited subset of git commands.
+	 */
+	else if (argc != 3 || strcmp(argv[1], "-c"))
+		die("What do you think I am? A shell?");
+
+	prog = argv[2];
+	if (!strncmp(prog, "git", 3) && isspace(prog[3]))
+		/* Accept "git foo" as if the caller said "git-foo". */
+		prog[3] = '-';
+
+	for (cmd = cmd_list ; cmd->name ; cmd++) {
+		int len = strlen(cmd->name);
+		char *arg;
+		if (strncmp(cmd->name, prog, len))
+			continue;
+		arg = NULL;
+		switch (prog[len]) {
+		case '\0':
+			arg = NULL;
+			break;
+		case ' ':
+			arg = prog + len + 1;
+			break;
+		default:
+			continue;
+		}
+		exit(cmd->exec(cmd->name, arg));
+	}
+	die("unrecognized command '%s'", prog);
+}