new-installer: Use `` instead of wfShellExec() like the old installer
This avoids runnin into the memory limitation of ulimit.sh when
auto-detecting if programs exist during installation. On my 64 bit
Debian system calling e.g. diff3 --version takes more than the default
10MB memory limit.
Can be reproduced as:
<?php
require 'maintenance/commandLine.inc';
$var = wfShellExec("/usr/bin/diff3 --version 2>&1");
echo "<$var>\n";
?>
Which on my system gives you:
php test.php
/home/avar/g/phase3/bin/ulimit4.sh: xmalloc: ../bash/variables.c:3446: cannot allocate 1270 bytes (53248 bytes allocated)
<>
The old installer just did ``, revert to this behavior until we raise
the default memory limit, or find some workaround for this issue.