Favoring the use of bash "Parameter Expansion" instead of "dirname" as

"dirname" has some cross-platform challenges.

- removed the superfluous stderr redirection
- switched out dirname for param expansion

bin/git-extras

@@ -3,9 +3,10 @@
 VERSION="1.5.1"
 
 update() {
-  local binfile=$(which git-extras 2>/dev/null)
-  local binpath=$(dirname $(dirname ${binfile} 2>/dev/null) 2>/dev/null)
+  local binfile=$(which git-extras)
+  local binpath=${binfile%/*/*}
   local orig=$PWD
+
   cd /tmp \
     && rm -fr ./git-extras \
     && git clone --depth 1 https://github.com/visionmedia/git-extras.git \