Revert "prompt: clean up strbuf usage"

This reverts commit 31b49d9b.

That commit taught do_askpass to hand ownership of our
buffer back to the caller rather than simply return a
pointer into our internal strbuf.  What it failed to notice,
though, was that our internal strbuf is static, because we
are trying to emulate the getpass() interface.

By handing off ownership, we created a memory leak that
cannot be solved. Sometimes git_prompt returns a static
buffer from getpass() (or our smarter git_terminal_prompt
wrapper), and sometimes it returns an allocated string from
do_askpass.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

prompt.c

@@ -22,6 +22,7 @@ static char *do_askpass(const char *cmd, const char *prompt)
 	if (start_command(&pass))
 		return NULL;
 
+	strbuf_reset(&buffer);
 	if (strbuf_read(&buffer, pass.out, 20) < 0)
 		err = 1;
 
@@ -38,7 +39,7 @@ static char *do_askpass(const char *cmd, const char *prompt)
 
 	strbuf_setlen(&buffer, strcspn(buffer.buf, "\r\n"));
 
-	return strbuf_detach(&buffer, NULL);
+	return buffer.buf;
 }
 
 char *git_prompt(const char *prompt, int flags)