patch 8.1.0729: there is a SourcePre autocommand event but not a SourcePost

Problem:    There is a SourcePre autocommand event but not a SourcePost.
Solution:   Add the SourcePost autocommand event. (closes #3739)

src/testdir/test_source.vim

@@ -0,0 +1,38 @@
+" Tests for the :source command.
+
+func Test_source_autocmd()
+  call writefile([
+	\ 'let did_source = 1',
+	\ ], 'Xsourced')
+  au SourcePre *source* let did_source_pre = 1
+  au SourcePost *source* let did_source_post = 1
+
+  source Xsourced
+
+  call assert_equal(g:did_source, 1)
+  call assert_equal(g:did_source_pre, 1)
+  call assert_equal(g:did_source_post, 1)
+
+  call delete('Xsourced')
+  au! SourcePre
+  au! SourcePost
+  unlet g:did_source
+  unlet g:did_source_pre
+  unlet g:did_source_post
+endfunc
+
+func Test_source_cmd()
+  au SourceCmd *source* let did_source = expand('<afile>')
+  au SourcePre *source* let did_source_pre = 2
+  au SourcePost *source* let did_source_post = 2
+
+  source Xsourced
+
+  call assert_equal(g:did_source, 'Xsourced')
+  call assert_false(exists('g:did_source_pre'))
+  call assert_equal(g:did_source_post, 2)
+
+  au! SourceCmd
+  au! SourcePre
+  au! SourcePost
+endfunc