cmd/update-reset: accept tap names as arguments

Currently, doing `brew update-reset homebrew/core` does nothing (not
even return an error). If you want to `update-reset` a given tap, you
must do (the equivalent of)

    brew update-reset "$(brew --repository owner/tap_name)"

This isn't very intuitive, so let's do a bit more work in argument
parsing so that the user can just pass a tap name instead of a path to a
tap.

Passing a path to a tap is also still supported.

Library/Homebrew/cmd/update-reset.sh

@@ -33,7 +33,22 @@ homebrew-update-reset() {
         [[ "${option}" == *d* ]] && HOMEBREW_DEBUG=1
         ;;
       *)
-        REPOS+=("${option}")
+        if [[ -d "${option}" ]]
+        then
+          REPOS+=("${option}")
+        else
+          local owner="${option%/*}"
+          local name="${option#*/}"
+          name="${name#homebrew-}" # Support both homebrew/core and homebrew/homebrew-core.
+
+          local repo_path="${HOMEBREW_LIBRARY}/Taps/${owner}/homebrew-${name}"
+          if [[ -d "${repo_path}" ]]
+          then
+            REPOS+=("${repo_path}")
+          else
+            odie "Could not find tap: ${option}"
+          fi
+        fi
         ;;
     esac
   done