Update .editorconfig and .gitattributes for postgresql.conf.sample.

This commit updates .editorconfig and .gitattributes in preparation
for a follow-up commit that will modify this file to use spaces
instead of tabs.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aReNUKdMgKxLqmq7%40nathan

.editorconfig

@@ -85,6 +85,12 @@ insert_final_newline = true
 indent_style = unset
 tab_width = unset
 
+[src/backend/utils/misc/postgresql.conf.sample]
+trim_trailing_whitespace = true
+insert_final_newline = true
+indent_style = space
+tab_width = unset
+
 [*.out]
 indent_style = unset
 indent_size = unset

.gitattributes

@@ -19,6 +19,7 @@ README.*	conflict-marker-size=48
 *.data						-whitespace
 contrib/pgcrypto/sql/pgp-armor.sql		whitespace=-blank-at-eol
 src/backend/catalog/sql_features.txt		whitespace=space-before-tab,blank-at-eof,-blank-at-eol
+src/backend/utils/misc/postgresql.conf.sample	whitespace=space-before-tab,trailing-space,tab-in-indent
 
 # Test output files that contain extra whitespace
 *.out					-whitespace