update: abbreviate hashes to at least 7 characters

Abbreviations will be longer if needed to preserve uniqueness.

This makes it more consistent with `git rev-parse --short`,
https://github.com/Homebrew/homebrew/commits, etc.

For example, instead of:

    Updated Homebrew from 40d1e9c2 to 90b9bdf4.

We see:

    Updated Homebrew from 40d1e9c to 90b9bdf.

See 0c48248b6555353c8d60b61973639c8423ce586c for the original
introduction of eight-character abbreviations.

1 files changed, 7 insertions, 2 deletions

diff --git a/Library/Homebrew/cmd/update.rb b/Library/Homebrew/cmd/update.rb
index 436175b25..f93224c8d 100644
--- a/Library/Homebrew/cmd/update.rb
+++ b/Library/Homebrew/cmd/update.rb
@@ -41,8 +41,9 @@ module Homebrew
     master_updater.pull!
     master_updated = master_updater.updated?
     if master_updated
-      puts "Updated Homebrew from #{master_updater.initial_revision[0, 8]} " \
-           "to #{master_updater.current_revision[0, 8]}."
+      initial_short = shorten_revision(master_updater.initial_revision)
+      current_short = shorten_revision(master_updater.current_revision)
+      puts "Updated Homebrew from #{initial_short} to #{current_short}."
     end
     report.update(master_updater.report)
 
@@ -133,6 +134,10 @@ module Homebrew
 
   private
 
+  def shorten_revision(revision)
+    `git rev-parse --short #{revision}`.chomp
+  end
+
   def git_init_if_necessary
     if Dir[".git/*"].empty?
       safe_system "git", "init"