Make git-summary take an optional commitish

This allows you to ask for a git summary based on a commitish, which is
very useful if you have a lot of project history. For example, if you
want a summary since the tag v42

$ git summary v42..

bin/git-summary

@@ -1,9 +1,11 @@
 #!/bin/sh
 
+COMMITISH=""
+test $# -ne 0 && COMMITISH=$@
 project=${PWD##*/}
 commit_count=`git log --oneline | wc -l | tr -d ' '`
 file_count=`git ls-files | wc -l | tr -d ' '`
-authors=`git shortlog -n -s`
+authors=`git shortlog -n -s $COMMITISH`
 
 echo
 echo " project: $project"
@@ -11,4 +13,4 @@ echo " commits: $commit_count"
 echo " files  : $file_count"
 echo " authors: "
 echo "$authors"
-echo
+echo