Add post-merge hook, related documentation, and tests.

The post-merge hook enables one to hook in for `git pull` operations in order
to check and/or change attributes of a work tree from the hook.  As an example,
it can be used in combination with a pre-commit hook to save/restore file
ownership and permissions data (or file ACLs) within the repository and
transparently update the working tree after a `git pull` operation.

Signed-off-by: Josh England <jjengla@sandia.gov>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-merge.sh

@@ -97,6 +97,19 @@ finish () {
 		fi
 		;;
 	esac
+
+	# Run a post-merge hook
+        if test -x "$GIT_DIR"/hooks/post-merge
+        then
+	    case "$squash" in
+	    t)
+                "$GIT_DIR"/hooks/post-merge 1
+		;;
+	    '')
+                "$GIT_DIR"/hooks/post-merge 0
+		;;
+	    esac
+        fi
 }
 
 merge_name () {