crash in git_attrs.git_attribute function

A recent change in this function sometimes causes a crash when
pushing a commit. This happens in a situation such as the following:

The user has a repository that looks like this:

     <root>/
        subdir/
           .gitattributes
           sub_sub_dir/
              .gitattributes
              some_file

If the user creates a commit which modifies some_file, the git_attribute
function will be passed a list of files which includes:

    subdir/sub_sub_dir/some_file

It will process that file by first looking at subdir/sub_sub_dir,
check if there is a .gitattributes in it (yes), so create that
directory, add it to the dirs_with_changes "list" so we know
we've already processed that directory. So far, so good.

Loop again, this time with os.path.dirname('subdir/sub_sub_dir'),
thus with 'subdir'. Is that directory in dirs_with_changes? No.
Does it have a .gitattribute in it? Yes.  So, rince-repeat, which
means first creating the subdir. That leads to the crash, since
we already just created it as a side-effect of creating its subdir.

This patch fixes the issue by simply remembering when a directory
has been created for a given file in filename_list.  Once that's
done, we know that the next iterations on the dir_path loop are
necessarily for parent directories, which have therefore been
created, even if they are not listed in the dirs_with_changes
"list" yet.

Another possible fix, which would be safer, would be to just
check the filesystem directly. But this code can be a hot-spot
in terms of performance, so we'll try really hard to avoid it.

Fixes P601-011.
Related to P531-036.

hooks/git_attrs.py

@@ -129,13 +129,16 @@ def git_attribute(commit_rev, filename_list, attr_name):
     for filename in filename_list:
         assert not os.path.isabs(filename)
         dir_path = filename
+        dir_created = False
         while dir_path:
             dir_path = os.path.dirname(dir_path)
             if dir_path in dirs_with_changes:
                 continue
             gitattributes_rel_file = os.path.join(dir_path, '.gitattributes')
             if cached_file_exists(commit_rev, gitattributes_rel_file):
-                os.makedirs(os.path.join(tmp_checkout_dir, dir_path))
+                if not dir_created:
+                    os.makedirs(os.path.join(tmp_checkout_dir, dir_path))
+                    dir_created = True
                 git.show("%s:%s" % (commit_rev, gitattributes_rel_file),
                          _outfile=os.path.join(tmp_checkout_dir,
                                                gitattributes_rel_file))

testsuite/tests/P601-011__gitattributes_handling/bare_repo_config

@@ -0,0 +1,4 @@
+[core]
+	repositoryformatversion = 0
+	filemode = true
+	bare = true

testsuite/tests/P601-011__gitattributes_handling/hooks_config

@@ -0,0 +1,3 @@
+[hooks]
+        from-domain = adacore.com
+        mailinglist = git-hooks-ci@example.com

testsuite/tests/P601-011__gitattributes_handling/test.py

@@ -0,0 +1,65 @@
+from support import *
+
+class TestRun(TestCase):
+    def test_push_commit_on_master(self):
+        """Try pushing one single-file commit on master.
+        """
+        cd ('%s/repo' % TEST_DIR)
+
+        # Push master to the `origin' remote.  The delta should be one
+        # commit with one file being modified.
+        p = Run('git push origin master'.split())
+        expected_out = """\
+remote: DEBUG: Content-Type: text/plain; charset="us-ascii"
+remote: MIME-Version: 1.0
+remote: Content-Transfer-Encoding: 7bit
+remote: From: Test Suite <testsuite@adacore.com>
+remote: To: git-hooks-ci@example.com
+remote: Bcc: file-ci@gnat.com
+remote: Subject: [repo] Minor reformatting.
+remote: X-Act-Checkin: repo
+remote: X-Git-Author: Joel Brobecker <brobecker@adacore.com>
+remote: X-Git-Refname: refs/heads/master
+remote: X-Git-Oldrev: 937ea8ac48994bdef9928dc6e246bb1d3efb8acd
+remote: X-Git-Newrev: 4b27b37bcd68460e2ec2ccd97a4e93660ed4cd16
+remote:
+remote: commit 4b27b37bcd68460e2ec2ccd97a4e93660ed4cd16
+remote: Author: Joel Brobecker <brobecker@adacore.com>
+remote: Date:   Wed Jun 1 09:38:20 2016 -0700
+remote:
+remote:     Minor reformatting.
+remote:
+remote: Diff:
+remote: ---
+remote:  gms/hello.adb | 1 +
+remote:  sec/README    | 3 +++
+remote:  2 files changed, 4 insertions(+)
+remote:
+remote: diff --git a/gms/hello.adb b/gms/hello.adb
+remote: index 9a6ff61..93db53d 100644
+remote: --- a/gms/hello.adb
+remote: +++ b/gms/hello.adb
+remote: @@ -1,4 +1,5 @@
+remote:  with Ada.Text_IO; use Ada.Text_IO;
+remote: +
+remote:  procedure Hello is
+remote:  begin
+remote:     Put_Line ("Hello World.");
+remote: diff --git a/sec/README b/sec/README
+remote: index a7e57ad..93f282d 100644
+remote: --- a/sec/README
+remote: +++ b/sec/README
+remote: @@ -1 +1,4 @@
+remote: +REPO's sec/ README:
+remote: +-------------------
+remote: +
+remote:  This directory contains interesting stuff.
+To ../bare/repo.git
+   937ea8a..4b27b37  master -> master
+"""
+
+        self.assertEqual(p.status, 0, p.image)
+        self.assertRunOutputEqual(p, expected_out)
+
+if __name__ == '__main__':
+    runtests()