update: make stashing local changes more robust

Users with local changes and without a configured Git identity won't be
able to update Homebrew via `brew update`, as the update will fail when
trying to stash the local modifications with `git stash`. They will be
unable to proceed until they follow Git's advice to configure their
identity or they manage to revert their local changes.

This change always sets a commit e-mail and name, avoiding this issue. A
nice bonus is that experienced Git users can see who created the stash
commit (identifying `brew update` as the author).

Fixes Homebrew/homebrew#46930.

Closes Homebrew/homebrew#46939.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/cmd/update.rb

@@ -225,7 +225,9 @@ class Updater
         puts "Stashing uncommitted changes to #{repository}."
         system "git", "status", "--short", "--untracked-files=all"
       end
-      safe_system "git", "stash", "save", "--include-untracked", *@quiet_args
+      safe_system "git", "-c", "user.email=brew-update@localhost",
+                         "-c", "user.name=brew update",
+                         "stash", "save", "--include-untracked", *@quiet_args
       safe_system "git", "reset", "--hard", *@quiet_args
       @stashed = true
     end