[PATCH] Improve the filemode trustability check

["Torsten Bögershausen" <tboegi@web.de>]

Some file systems do not support the executable bit:
a) The user executable bit is always 0, e.g. VFAT mounted with -onoexec
b) The user executable bit is always 1, e.g. cifs mounted with -ofile_mode=0755
c) There are system where user executable bit is 1 even if it should be 0
   like b), but the file mode can be maintained locally. chmod -x changes the
   file mode from 0766 to 0666, until the file system is unmounted and
   remounted and the file mode is 0766 again.
   This been observed when a Windows machine with NTFS exports a share to
   Mac OS X via smb or afp.

Case a) and b) are handled by the current code.
Case c) qualifies as "non trustable executable bit" and core.filemode
should be false, but this is not done.

Solution:
Detect when ".git/config" has the user executable bit set after
creat(".git/config", 0666) and set core.filemode to false.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
---

This should go on top of "mh/config-flip-xbit-back-after-checking"

Michael, thanks for the test case.
And no, I havent seen any systems with behaving like d)

 builtin/init-db.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/builtin/init-db.c b/builtin/init-db.c
index aab44d2..9b50dde 100644
--- a/builtin/init-db.c
+++ b/builtin/init-db.c
@@ -252,10 +252,10 @@ static int create_default_files(const char *template_path)
 	filemode = TEST_FILEMODE;
 	if (TEST_FILEMODE && !lstat(path, &st1)) {
 		struct stat st2;
-		filemode = (!chmod(path, st1.st_mode ^ S_IXUSR) &&
+		filemode = (!chmod(path, st1.st_mode | S_IXUSR) &&
 				!lstat(path, &st2) &&
 				st1.st_mode != st2.st_mode &&
-				!chmod(path, st1.st_mode));
+				!chmod(path, st1.st_mode & (~S_IXUSR)));
 	}
 	git_config_set("core.filemode", filemode ? "true" : "false");
 
-- 
1.9.1.dirty