Reuse tempfile base names

This reverts "Use fresh temp files for every diff" [1].  At the time I
did not realise that Vim never checks the existence of its temp
directory after creating it at startup; I thought that each call to
tempname() generated a fresh file on disk.  In fact tempname() simply
generates a string path.

Therefore there is no point calling tempname() for every diff.  Doing so
only serves to generate more files in the temp directory than necessary.

[1] dab840b

autoload/gitgutter/diff.vim

@@ -12,6 +12,8 @@ endfunction
 
 let s:c_flag = s:git_supports_command_line_config_override()
 
+let s:temp_from = tempname()
+let s:temp_buffer = tempname()
 let s:counter = 0
 
 " Returns a diff of the buffer against the index or the working tree.
@@ -75,9 +77,6 @@ function! gitgutter#diff#run_diff(bufnr, from, preserve_full_diff) abort
     throw 'gitgutter not tracked'
   endif
 
-  let temp_from = tempname()
-  let temp_buffer = tempname()
-
   " Wrap compound commands in parentheses to make Windows happy.
   " bash doesn't mind the parentheses.
   let cmd = '('
@@ -90,7 +89,7 @@ function! gitgutter#diff#run_diff(bufnr, from, preserve_full_diff) abort
   " second gitgutter#process_buffer() writing the file (synchronously, below)
   " and the first gitgutter#process_buffer()'s async job reading it (with
   " git-diff).
-  let buff_file = temp_buffer.'.'.a:bufnr
+  let buff_file = s:temp_buffer.'.'.a:bufnr
 
   " Add a counter to avoid a similar race with two quick writes of the same buffer.
   " Use a modulus greater than a maximum reasonable number of visible buffers.
@@ -110,7 +109,7 @@ function! gitgutter#diff#run_diff(bufnr, from, preserve_full_diff) abort
     " Without the buffer number, from_file would have a race in the shell
     " between the second process writing it (with git-show) and the first
     " reading it (with git-diff).
-    let from_file = temp_from.'.'.a:bufnr
+    let from_file = s:temp_from.'.'.a:bufnr
 
     " Add a counter to avoid a similar race with two quick writes of the same buffer.
     let from_file .= '.'.s:counter