git_validate: better error with checkout being ahead/diverged

The `--count` and `--left-right` options seem to be in Git since before
1.7 already.

The error message is improved, and distinguishes between only being
ahead (which is the case if you could push), or being diverged (ahead
and defined).

I think it could be bad to accidentally clean/delete a local checkout
which is ahead, and therefore PlugClean is not mentioned then, which
appears to trigger it to show up in :PlugClean.

plug.vim

@@ -2013,10 +2013,24 @@ function! s:git_validate(spec, check_branch)
               \ branch, a:spec.branch)
       endif
       if empty(err)
-        let commits = len(s:lines(s:system(printf('git rev-list origin/%s..HEAD', a:spec.branch), a:spec.dir)))
+        let [ahead, behind] = split(s:lastline(s:system(printf(
-        if !v:shell_error && commits
+              \ 'git rev-list --count --left-right HEAD...origin/%s',
-          let err = join([printf('Diverged from origin/%s by %d commit(s).', a:spec.branch, commits),
+              \ a:spec.branch), a:spec.dir)), '\t')
-                        \ 'Reinstall after PlugClean.'], "\n")
+        if !v:shell_error && ahead
+          if behind
+            let base_error = printf(
+                  \ 'Diverged from origin/%%s (%%d commit(s) ahead and %d commit(s) behind!', behind)
+          else
+            let base_error = 'Ahead of origin/%s by %d commit(s).'
+          endif
+          let err = join([printf(base_error, a:spec.branch, ahead),
+                \ 'Can you push it yourself?',
+                \ 'Otherwise try reinstalling it after cleaning it manually.'], "\n")
+          " Only mention PlugClean if diverged, otherwise it's likely to be
+          " pushable (and probably not that messed up).
+          if behind
+            let err .= "\nTry PlugClean after backing it up."
+          endif
         endif
       endif
     endif