transport-helper: avoid reading past end-of-string

transport-helper: avoid reading past end-of-string

We detect the "import-marks" capability by looking for that
string, but _without_ a trailing space. Then we skip past it
using strlen("import-marks "), with a space. So if a remote
helper gives us exactly "import-marks", we will read past
the end-of-string by one character.

This is unlikely to be a problem in practice, because such
input is malformed in the first place, and because there is
a good chance that the string has an extra NUL terminator
one character after the original (because it formerly had a
newline in it that we parsed off).

We can fix it by using skip_prefix with "import-marks ",
with the space. The other form appears to be a typo from
a515ebe (transport-helper: implement marks location as
capability, 2011-07-16); "import-marks" has never existed
without an argument, and it should match the "export-marks"
definition above.

Speaking of which, we can also use skip_prefix in a few
other places while we are in the function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>