Add git-shell.

This adds a very git specific restricted shell, that can be
added to /etc/shells and set to the pw_shell in the /etc/passwd
file, to give users ability to push into repositories over ssh
without giving them full interactive shell acount.

[jc: I updated Linus' patch to match what the current sq_quote()
 does.]

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

quote.h

@@ -31,6 +31,12 @@
 extern char *sq_quote(const char *src);
 extern size_t sq_quote_buf(char *dst, size_t n, const char *src);
 
+/* This unwraps what sq_quote() produces in place, but returns
+ * NULL if the input does not look like what sq_quote would have
+ * produced.
+ */
+extern char *sq_dequote(char *);
+
 extern int quote_c_style(const char *name, char *outbuf, FILE *outfp,
 			 int nodq);
 extern char *unquote_c_style(const char *quoted, const char **endp);