lsp: fix applyTextEdits

Fix applyTextEdits to correctly calculate the prefix of the replacement
text.

Fixes fatih#3231

Author: bhcleek

autoload/go/lsp.vim

@@ -1741,15 +1741,14 @@ function s:applyTextEdits(bufnr, msg) abort
       continue
     endif
 
-    let l:startcontent = getline(l:startline)
-    let l:preSliceEnd = 0
+    " Assume that l:startcontent will be an empty string. When the replacement
+    " is not at the beginning of the line, then l:startcontent must be what
+    " comes before the start position on the start line.
+    let l:startcontent = ''
     if l:msg.range.start.character > 0
+      let l:startcontent = getline(l:startline)
       let l:preSliceEnd = go#lsp#lsp#PositionOf(l:startcontent, l:msg.range.start.character-1) - 1
       let l:startcontent = l:startcontent[:l:preSliceEnd]
-    elseif l:endline == l:startline && (l:msg.range.end.character == 0 || l:msg.range.start.character == 0)
-      " l:startcontent should be the empty string when l:text is a
-      " replacement at the beginning of the line.
-      let l:startcontent = ''
     endif
 
     let l:endcontent = getline(l:endline)
@@ -1820,7 +1819,7 @@ function! s:textEditLess(left, right) abort
     endif
   endif
 
-  " return 0, because a:left an a:right refer to the same position.
+  " return 0, because a:left and a:right refer to the same position.
   return 0
 endfunction