ENH: Avoid tag updating when ctags output has identical fingerprint.

We can avoid the lengthy and blocking update of the tags database when the ctags output returns the same information as before; i.e. nothing tag-relevant has been changed since the last update.
Since 7.3.816, Vim has a sha256() function that allows us to quickly calculate a fingerprint over the ctags output. When that is not available, only handle the special case of a (covered) file that has no tags at all (as calculating a hash in Vimscript is costly and would probably defeat the intended speedup).

autoload/xolox/easytags.vim

@@ -99,8 +99,11 @@ function! xolox#easytags#update(silent, filter_tags, filenames) " {{{2
     let tagsfile = xolox#easytags#get_tagsfile()
     let firstrun = !filereadable(tagsfile)
     let cmdline = s:prep_cmdline(cfile, tagsfile, firstrun, a:filenames, context)
-    let output = s:run_ctags(starttime, cfile, tagsfile, firstrun, cmdline)
+    let [output, has_updates] = s:run_ctags(starttime, cfile, tagsfile, firstrun, cmdline)
     if !firstrun
+      if !has_updates
+        return 1
+      endif
       if have_args && !empty(g:easytags_by_filetype)
         " TODO Get the headers from somewhere?!
         call s:save_by_filetype(a:filter_tags, [], output, context)
@@ -201,12 +204,41 @@ function! s:prep_cmdline(cfile, tagsfile, firstrun, arguments, context) " {{{3
   return have_args ? join(cmdline) : ''
 endfunction
 
+if exists('*sha256')
+function! s:get_fingerprint(cfile, output)
+  return sha256(a:output)
+endfunction
+else
+function! s:get_fingerprint(cfile, output)
+  " Don't want to re-implement a costly hashing function in Vimscript. Just
+  " handle files that never had any tags.
+  if empty(a:output)
+    return get(s:fingerprints, a:cfile, 1)
+  else
+    return ''
+  endif
+endfunction
+endif
+
+let s:fingerprints = {}
+function! s:has_updates(cfile, output)
+  let fingerprint = s:get_fingerprint(a:cfile, a:output)
+  if ! empty(fingerprint) && get(s:fingerprints, a:cfile, '') ==# fingerprint
+    return 0
+  endif
+
+  let s:fingerprints[a:cfile] = fingerprint
+  return 1
+endfunction
+
 function! s:run_ctags(starttime, cfile, tagsfile, firstrun, cmdline) " {{{3
   let lines = []
+  let has_updates = 1
   if a:cmdline != ''
     call xolox#misc#msg#debug("easytags.vim %s: Executing %s.", g:xolox#easytags#version, a:cmdline)
     try
       let lines = xolox#shell#execute(a:cmdline, 1)
+      let has_updates = a:firstrun || s:has_updates(a:cfile, join(lines, "\n"))
     catch /^Vim\%((\a\+)\)\=:E117/
       " Ignore missing shell.vim plug-in.
       let output = system(a:cmdline)
@@ -215,6 +247,7 @@ function! s:run_ctags(starttime, cfile, tagsfile, firstrun, cmdline) " {{{3
         throw printf(msg, fnamemodify(a:tagsfile, ':~'), strtrans(output))
       endif
       let lines = split(output, "\n")
+      let has_updates = a:firstrun || s:has_updates(a:cfile, output)
     endtry
     if a:firstrun
       if a:cfile != ''
@@ -225,7 +258,7 @@ function! s:run_ctags(starttime, cfile, tagsfile, firstrun, cmdline) " {{{3
       return []
     endif
   endif
-  return xolox#easytags#parse_entries(lines)
+  return [xolox#easytags#parse_entries(lines), has_updates]
 endfunction
 
 function! s:filter_merge_tags(filter_tags, tagsfile, output, context) " {{{3