submodule.c: convert is_submodule_modified to use strbuf_getwholeline

submodule.c: convert is_submodule_modified to use strbuf_getwholeline

Instead of implementing line reading yet again, make use of our beautiful
library function to read one line.  By using strbuf_getwholeline instead
of strbuf_read, we avoid having to allocate memory for the entire child
process output at once.  That is, we limit maximum memory usage.
Also we can start processing the output as it comes in, no need to
wait for all of it.

Once we know all information that we care about, we can terminate
the child early. In that case we do not care about its exit code as well.
By just closing our side of the pipe the child process will get a SIGPIPE
signal, which it will not report nor do we report it in finish_command,
ac78663b0d (run-command: don't warn on SIGPIPE deaths, 2015-12-29).

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>