Fix Git ignore rules for tmp/, shared/, and plugins/
What does this MR do?
The old rules meant that any future changes to these directories would
require the use of git add -f to be staged. This is problematic for
the upcoming FOSS-only mirror of GitLab. Any changes to be synced to
this mirror are staged using git add --all ., which due to the Git
ignore rules would not add the tmp/ and shared/ directories. This then
leads to various specs failing because they expect certain
sub-directories of these directories to be available.
This commit changes the rules so that we include the .gitkeep files, ignore other files, and completely ignore anything in tmp/test. This requires that the rules are specified in tmp/.gitignore and shared/.gitignore, instead of the top-level .gitignore, without this I could not get Git to include the .gitkeep files properly.