strbuf.c: add strbuf_join_argv()

Implement `strbuf_join_argv()` to join arguments
into a strbuf.

Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -288,6 +288,13 @@ static inline void strbuf_addstr(struct strbuf *sb, const char *s)
  */
 void strbuf_addbuf(struct strbuf *sb, const struct strbuf *sb2);
 
+/**
+ * Join the arguments into a buffer. `delim` is put between every
+ * two arguments.
+ */
+const char *strbuf_join_argv(struct strbuf *buf, int argc,
+			     const char **argv, char delim);
+
 /**
  * This function can be used to expand a format string containing
  * placeholders. To that end, it parses the string and calls the specified