Adds a do not prefill :GoRename setting

Added a setting to prevent :GoRename from prefilling. Also if the to
identifer is empty, just return. This way if you run :GoRename but
decide to not rename, simply press escape.

autoload/go/rename.vim

@@ -2,21 +2,32 @@ if !exists("g:go_gorename_bin")
     let g:go_gorename_bin = "gorename"
 endif
 
+if !exists("g:go_gorename_prefill")
+    let g:go_gorename_prefill = 1
+endif
+
 function! go#rename#Rename(bang, ...)
     let to = ""
     if a:0 == 0
         let from = expand("<cword>")
         let ask = printf("vim-go: rename '%s' to: ", from)
-        let to = input(ask, from)
-        redraw
+        if g:go_gorename_prefill
+            let to = input(ask, from)
+        else
+            let to = input(ask)
+        endif
+        redraw!
+        if empty(to)
+            return
+        endif
     else
         let to = a:1
     endif
 
     "return with a warning if the bin doesn't exist
-    let bin_path = go#path#CheckBinPath(g:go_gorename_bin) 
-    if empty(bin_path) 
-        return 
+    let bin_path = go#path#CheckBinPath(g:go_gorename_bin)
+    if empty(bin_path)
+        return
     endif
 
     let fname = expand('%:p')

doc/vim-go.txt

@@ -1054,6 +1054,13 @@ It is by default set to edit.
 >
   let g:go_alternate_mode = "edit"
 <
+                                                       *g:go_gorename_prefill*
+
+Specifies whether |:GoRename| prefills the new identifer name with the
+word under the cursor. By default is is on.
+>
+  let g:go_gorename_prefill = 1
+<
 
 
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