Item13135: Add a "gitignore" target to BuildContrib

This also fixes a minor issue to the MANIFEST.  It was including the
TIDY file.

After building and validating the MANIFEST file,  the gitignore target
builds a candidate .gitignore based upon files listed in the MANIFEST
that are not known to git.
Item11267: check_manifest incomplete changes

Forgot that check_manifest-git was merged into check_manifest.pl. Change
shell script to call the right tool.  Also check_manifest was not
ignoring the .gitignore file.