revision: small readability improvement for reading from stdin

revision: small readability improvement for reading from stdin

The code that reads lines from standard input manually compares whether
the read line matches "--", which is a bit awkward to read. Furthermore,
we're about to extend the code to also support reading pseudo-options
via standard input, requiring more elaborate handling of lines with a
leading dash.

Refactor the code by hoisting out the check for "--" outside of the
block that checks for a leading dash.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>