update: use lock

Closes #181.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/update.sh

@@ -5,6 +5,8 @@
 #:    If `--merge` is specified then `git merge` is used to include updates
 #:      (rather than `git rebase`).
 
+source "$HOMEBREW_LIBRARY/Homebrew/utils/lock.sh"
+
 brew() {
   "$HOMEBREW_BREW_FILE" "$@"
 }
@@ -307,6 +309,9 @@ EOS
   # ensure GIT_CONFIG is unset as we need to operate on .git/config
   unset GIT_CONFIG
 
+  # only allow one instance of brew update
+  lock update
+
   chdir "$HOMEBREW_REPOSITORY"
   git_init_if_necessary
   # rename Taps directories