download_strategy: ensure fixed commit hash length

The length of the commit hash returned for `--short` can vary depending
on user configuration. Make sure this works independently of what might
have been configured via a user's `.gitconfig`. This also fixes the
failing `GitDownloadStrategyTests#test_last_commit` test for such users.

Library/Homebrew/download_strategy.rb

@@ -592,7 +592,7 @@ def source_modified_time
   end
 
   def last_commit
-    Utils.popen_read("git", "--git-dir", git_dir ,"rev-parse", "--short", "HEAD").chomp
+    Utils.popen_read("git", "--git-dir", git_dir, "rev-parse", "--short=7", "HEAD").chomp
   end
 
   private