Only remove undo point if s:RunRustfmt is executed from BufWritePre

Otherwise, we remove unintended undo points.

We can use the already existing parameter to figure out whether we are
executing from BufWritePre context.

Fixes #358. Issue existed since #33c3e214a606, introduced by #111.

Author: da-x

autoload/rustfmt.vim

@@ -110,7 +110,7 @@ function! s:DeleteLines(start, end) abort
     silent! execute a:start . ',' . a:end . 'delete _'
 endfunction
 
-function! s:RunRustfmt(command, tmpname, fail_silently)
+function! s:RunRustfmt(command, tmpname, from_writepre)
     mkview!
 
     let l:stderr_tmpname = tempname()
@@ -147,8 +147,10 @@ function! s:RunRustfmt(command, tmpname, fail_silently)
 
     let l:open_lwindow = 0
     if v:shell_error == 0
-        " remove undo point caused via BufWritePre
-        try | silent undojoin | catch | endtry
+        if a:from_writepre
+            " remove undo point caused via BufWritePre
+            try | silent undojoin | catch | endtry
+        endif
 
         if a:tmpname ==# ''
             let l:content = l:out
@@ -168,7 +170,7 @@ function! s:RunRustfmt(command, tmpname, fail_silently)
             call setloclist(0, [])
             let l:open_lwindow = 1
         endif
-    elseif g:rustfmt_fail_silently == 0 && a:fail_silently == 0
+    elseif g:rustfmt_fail_silently == 0 && !a:from_writepre
         " otherwise get the errors and put them in the location list
         let l:errors = []
 
@@ -222,12 +224,12 @@ function! rustfmt#FormatRange(line1, line2)
     let l:tmpname = tempname()
     call writefile(getline(1, '$'), l:tmpname)
     let command = s:RustfmtCommandRange(l:tmpname, a:line1, a:line2)
-    call s:RunRustfmt(command, l:tmpname, 0)
+    call s:RunRustfmt(command, l:tmpname, v:false)
     call delete(l:tmpname)
 endfunction
 
 function! rustfmt#Format()
-    call s:RunRustfmt(s:RustfmtCommand(), '', 0)
+    call s:RunRustfmt(s:RustfmtCommand(), '', v:false)
 endfunction
 
 function! rustfmt#Cmd()
@@ -255,7 +257,7 @@ function! rustfmt#PreWrite()
         return
     endif
 
-    call s:RunRustfmt(s:RustfmtCommand(), '', 1)
+    call s:RunRustfmt(s:RustfmtCommand(), '', v:true)
 endfunction