Put .gitattributes under Pantheon upstream control

This way, .gitattributes will be checked out into developer working
copies and will have its intended effect, rather than being hidden in
the build artifacts.

Also link to pantheon.yml documentation

.gitattributes

@@ -0,0 +1,61 @@
+# Drupal git normalization
+# @see https://www.kernel.org/pub/software/scm/git/docs/gitattributes.html
+# @see https://www.drupal.org/node/1542048
+
+# Normally these settings would be done with macro attributes for improved
+# readability and easier maintenance. However macros can only be defined at the
+# repository root directory. Drupal avoids making any assumptions about where it
+# is installed.
+
+# Define text file attributes.
+# - Treat them as text.
+# - Ensure no CRLF line-endings, neither on checkout nor on checkin.
+# - Detect whitespace errors.
+#   - Exposed by default in `git diff --color` on the CLI.
+#   - Validate with `git diff --check`.
+#   - Deny applying with `git apply --whitespace=error-all`.
+#   - Fix automatically with `git apply --whitespace=fix`.
+
+*.config  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.css     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.dist    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.engine  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.html    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=html
+*.inc     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.install text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.js      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.json    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.lock    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.map     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.md      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.module  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.php     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.po      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.profile text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.script  text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.sh      text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.sql     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.svg     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.theme   text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2 diff=php
+*.twig    text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.txt     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.xml     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+*.yml     text eol=lf whitespace=blank-at-eol,-blank-at-eof,-space-before-tab,tab-in-indent,tabwidth=2
+
+# Define binary file attributes.
+# - Do not treat them as text.
+# - Include binary diff in patches instead of "binary files differ."
+*.eot     -text diff
+*.exe     -text diff
+*.gif     -text diff
+*.gz      -text diff
+*.ico     -text diff
+*.jpeg    -text diff
+*.jpg     -text diff
+*.otf     -text diff
+*.phar    -text diff
+*.png     -text diff
+*.svgz    -text diff
+*.ttf     -text diff
+*.woff    -text diff
+*.woff2   -text diff

.gitignore

@@ -121,7 +121,6 @@ upstream/web/
 /web/sites/example.sites.php
 /web/themes/README.txt
 /.drush-lock-update
-/.gitattributes
 
 # Other common rules
 # ------------------

composer.json

@@ -50,7 +50,8 @@
             },
             "file-mapping": {
                 "[project-root]/.editorconfig": false,
-                "[project-root]/pantheon.upstream.yml": false
+                "[project-root]/pantheon.upstream.yml": false,
+                "[project-root]/.gitattributes": false
             }
         }
     },

pantheon.upstream.yml

@@ -1,5 +1,6 @@
 api_version: 1
 web_docroot: true
+# See https://pantheon.io/docs/pantheon-yml/#enforce-https--hsts for valid values.
 enforce_https: transitional
 php_version: 7.4
 drush_version: 10