Support cvs via git-shell

This adds cvs support to the git-shell; You can now give new users
a restricted git-shell and they still can commit via git's cvs
emulator.

Note that either the gecos information must be accurate, or you must
provide a $HOME/.gitconfig with the appropriate user credentials.
Since the git-shell is too restricted to allow the user to do it
(on purpose!), it is up to the administrator to take care of that.

Based on an idea by Jan Wielemaker.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

shell.c

@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "quote.h"
 #include "exec_cmd.h"
+#include "strbuf.h"
 
 static int do_generic_cmd(const char *me, char *arg)
 {
@@ -18,12 +19,34 @@ static int do_generic_cmd(const char *me, char *arg)
 	return execv_git_cmd(my_argv);
 }
 
+static int do_cvs_cmd(const char *me, char *arg)
+{
+	const char *cvsserver_argv[3] = {
+		"cvsserver", "server", NULL
+	};
+	const char *oldpath = getenv("PATH");
+	struct strbuf newpath = STRBUF_INIT;
+
+	if (!arg || strcmp(arg, "server"))
+		die("git-cvsserver only handles server: %s", arg);
+
+	strbuf_addstr(&newpath, git_exec_path());
+	strbuf_addch(&newpath, ':');
+	strbuf_addstr(&newpath, oldpath);
+
+	setenv("PATH", strbuf_detach(&newpath, NULL), 1);
+
+	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 },
 };
 
@@ -32,8 +55,10 @@ int main(int argc, char **argv)
 	char *prog;
 	struct commands *cmd;
 
+	if (argc == 2 && !strcmp(argv[1], "cvs server"))
+		argv--;
 	/* We want to see "-c cmd args", and nothing else */
-	if (argc != 3 || strcmp(argv[1], "-c"))
+	else if (argc != 3 || strcmp(argv[1], "-c"))
 		die("What do you think I am? A shell?");
 
 	prog = argv[2];