[PATCH] Make sq_expand() available as sq_quote().

A useful shell safety helper sq_expand() was hidden as a static
function in diff.c.  Extract it out and make it available as
sq_quote().

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

quote.h

@@ -0,0 +1,26 @@
+#ifndef QUOTE_H
+#define QUOTE_H
+
+
+/* Help to copy the thing properly quoted for the shell safety.
+ * any single quote is replaced with '\'', and the whole thing
+ * is enclosed in a single quote pair.
+ *
+ * For example, if you are passing the result to system() as an
+ * argument:
+ *
+ * sprintf(cmd, "foobar %s %s", sq_quote(arg0), sq_quote(arg1))
+ *
+ * would be appropriate.  If the system() is going to call ssh to
+ * run the command on the other side:
+ *
+ * sprintf(cmd, "git-diff-tree %s %s", sq_quote(arg0), sq_quote(arg1));
+ * sprintf(rcmd, "ssh %s %s", sq_quote(host), sq_quote(cmd));
+ *
+ * Note that the above examples leak memory!  Remember to free result from
+ * sq_quote() in a real application.
+ */
+
+char *sq_quote(const char *src);
+
+#endif