charset: if we changed fenc from '' to enc, set nomodified

If the buffer had no fileencoding, and we changed fenc to match the Vim-wide
encoding, we haven't actually changed the file.  However, we have set
modified on the buffer by virtue of assigning fenc.  In this case, set
nomodified since we didn't actually change anything.

This affects new buffers created during the VimEnter autocmd.

plugin/editorconfig.vim

@@ -403,7 +403,45 @@ endfunction " }}}2
 
 " }}}1
 
-function! s:ApplyConfig(bufnr, config) abort " Set the buffer options {{{1
+" Set the buffer options {{{1
+function! s:SetCharset(bufnr, charset) abort " apply config['charset']
+
+    " Remember the buffer's state so we can set `nomodifed` at the end
+    " if appropriate.
+    let l:orig_fenc = getbufvar(a:bufnr, "&fileencoding")
+    let l:orig_enc = getbufvar(a:bufnr, "&encoding")
+    let l:orig_modified = getbufvar(a:bufnr, "&modified")
+
+    if a:charset == "utf-8"
+        call setbufvar(a:bufnr, '&fileencoding', 'utf-8')
+        call setbufvar(a:bufnr, '&bomb', 0)
+    elseif a:charset == "utf-8-bom"
+        call setbufvar(a:bufnr, '&fileencoding', 'utf-8')
+        call setbufvar(a:bufnr, '&bomb', 1)
+    elseif a:charset == "latin1"
+        call setbufvar(a:bufnr, '&fileencoding', 'latin1')
+        call setbufvar(a:bufnr, '&bomb', 0)
+    elseif a:charset == "utf-16be"
+        call setbufvar(a:bufnr, '&fileencoding', 'utf-16be')
+        call setbufvar(a:bufnr, '&bomb', 1)
+    elseif a:charset == "utf-16le"
+        call setbufvar(a:bufnr, '&fileencoding', 'utf-16le')
+        call setbufvar(a:bufnr, '&bomb', 1)
+    endif
+
+    let l:new_fenc = getbufvar(a:bufnr, "&fileencoding")
+
+    " If all we did was change the fileencoding from the default to a copy
+    " of the default, we didn't actually modify the file.
+    if !l:orig_modified && (l:orig_fenc ==# '') && (l:new_fenc ==# l:orig_enc)
+        if g:EditorConfig_verbose
+            echo 'Setting nomodified on buffer ' . a:bufnr
+        endif
+        call setbufvar(a:bufnr, '&modified', 0)
+    endif
+endfunction
+
+function! s:ApplyConfig(bufnr, config) abort
     if g:EditorConfig_verbose
         echo 'Options: ' . string(a:config)
     endif
@@ -462,22 +500,7 @@ function! s:ApplyConfig(bufnr, config) abort " Set the buffer options {{{1
 
     if s:IsRuleActive('charset', a:config) &&
                 \ getbufvar(a:bufnr, '&modifiable')
-        if a:config["charset"] == "utf-8"
-            call setbufvar(a:bufnr, '&fileencoding', 'utf-8')
-            call setbufvar(a:bufnr, '&bomb', 0)
-        elseif a:config["charset"] == "utf-8-bom"
-            call setbufvar(a:bufnr, '&fileencoding', 'utf-8')
-            call setbufvar(a:bufnr, '&bomb', 1)
-        elseif a:config["charset"] == "latin1"
-            call setbufvar(a:bufnr, '&fileencoding', 'latin1')
-            call setbufvar(a:bufnr, '&bomb', 0)
-        elseif a:config["charset"] == "utf-16be"
-            call setbufvar(a:bufnr, '&fileencoding', 'utf-16be')
-            call setbufvar(a:bufnr, '&bomb', 1)
-        elseif a:config["charset"] == "utf-16le"
-            call setbufvar(a:bufnr, '&fileencoding', 'utf-16le')
-            call setbufvar(a:bufnr, '&bomb', 1)
-        endif
+        call s:SetCharset(a:bufnr, a:config["charset"])
     endif
 
     augroup editorconfig_trim_trailing_whitespace