Fix Git ignore for tmp/, shared/, and plugins/

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.

.gitignore

@@ -61,11 +61,9 @@ eslint-report.html
 /shared/artifacts/
 /rails_best_practices_output.html
 /tags
-/tmp/*
 /vendor/bundle/*
 /vendor/gitaly-ruby
 /builds*
-/shared/*
 /.gitlab_workhorse_secret
 /webpack-report/
 /knapsack/
@@ -73,7 +71,6 @@ eslint-report.html
 /locale/**/LC_MESSAGES
 /locale/**/*.time_stamp
 /.rspec
-/plugins/*
 /.gitlab_pages_secret
 /.gitlab_smime_key
 /.gitlab_smime_cert

plugins/.gitignore

+*
+!*/
+!.gitignore
+!.gitkeep
+!examples/*

shared/.gitignore

+*
+!*/
+!.gitignore
+!.gitkeep

tmp/.gitignore

+*
+!*/
+!.gitignore
+!.gitkeep
+/tests/