dir.c: git-status: avoid is_excluded checks for tracked files

Checking if a file is in the index is much faster (hashtable lookup) than
checking if the file is excluded (linear search over exclude patterns).

Skip is_excluded checks for files: move the cache_name_exists check from
treat_file to treat_one_path and return early if the file is tracked.

This can safely be done as all other code paths also return path_ignored
for tracked files, and dir_add_ignored skips tracked files as well.

There's just one line left in treat_file, so move this to treat_one_path
as well.

Here's some performance data for git-status from the linux and WebKit
repos (best of 10 runs on a Debian Linux on SSD, core.preloadIndex=true):

       |    status      | status --ignored
       | linux | WebKit | linux | WebKit
-------+-------+--------+-------+---------
before | 0.218 |  1.583 | 0.321 |  2.579
after  | 0.156 |  0.988 | 0.202 |  1.279
gain   | 1.397 |  1.602 | 1.589 |  2.016

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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