1.6: add some .gitignore entries, including some Buildkite-related .gitignore entries (#47952)

Author: DilumAluthge

.gitignore

@@ -22,6 +22,7 @@
 *.so
 *.dylib
 *.dSYM
+*.h.gen
 *.jl.cov
 *.jl.*.cov
 *.jl.mem
@@ -32,9 +33,13 @@
 .DS_Store
 .idea/*
 .vscode/*
+*.heapsnapshot
 
-# Buildkite: cryptic plugin
-# Ignore the unencrypted repo_key
+# Buildkite: Ignore the entire .buildkite directory
+/.buildkite
+
+# Buildkite: Ignore the unencrypted repo_key
 repo_key
-# Ignore any agent keys (public or private) we have stored
+
+# Buildkite: Ignore any agent keys (public or private) we have stored
 agent_key*