[PATCH] Improve vcs_info example for ahead/behind git commits

[Tim Lee <progscriptclone@gmail.com>]

This patch improves the example hook that shows +N/-N when the
local git branch is ahead-of or behind the remote HEAD.

Improvements:

* Use git-rev-list's `--count` option instead of piping to `wc -l`.
  `--count` has been available since git 1.7.2
  (https://github.com/git/git/commit/f69c501832ecd6880602c55565508e70c3a013d5)
* Remove unnecessary use of `${hook_com[branch]}` because `@{upstream}`
  defaults to the current branch when no branch name is provided.
  `@{upstream}` was introduced in git 1.7.0
  (https://github.com/git/git/commit/28fb84382b0eb728534dbe2972bbfec3f3d83dd9)


diff --git a/Misc/vcs_info-examples b/Misc/vcs_info-examples
index 94b8a7b5e..36d4d3bf8 100644
--- a/Misc/vcs_info-examples
+++ b/Misc/vcs_info-examples
@@ -179,14 +179,18 @@ function +vi-git-st() {
     local ahead behind
     local -a gitstatus

-    # for git prior to 1.7
+    # for git prior to 1.7.0
     # ahead=$(git rev-list origin/${hook_com[branch]}..HEAD | wc -l)
-    ahead=$(git rev-list ${hook_com[branch]}@{upstream}..HEAD 2>/dev/null | wc -l)
+    # for git 1.7.0 and 1.7.1
+    # ahead=$(git rev-list @{upstream}..HEAD 2>/dev/null | wc -l)
+    ahead=$(git rev-list --count @{upstream}..HEAD 2>/dev/null)
     (( $ahead )) && gitstatus+=( "+${ahead}" )

-    # for git prior to 1.7
+    # for git prior to 1.7.0
     # behind=$(git rev-list HEAD..origin/${hook_com[branch]} | wc -l)
-    behind=$(git rev-list HEAD..${hook_com[branch]}@{upstream} 2>/dev/null | wc -l)
+    # for git 1.7.0 and 1.7.1
+    # behind=$(git rev-list HEAD..@{upstream} 2>/dev/null | wc -l)
+    behind=$(git rev-list --count HEAD..@{upstream} 2>/dev/null)
     (( $behind )) && gitstatus+=( "-${behind}" )

     hook_com[misc]+=${(j:/:)gitstatus}