create_default_files(): don't set u+x bit on $GIT_DIR/config

Since time immemorial, the test of whether to set "core.filemode" has
been done by trying to toggle the u+x bit on $GIT_DIR/config and then
testing whether the change "took". I find it somewhat odd to use the
config file for this test, but whatever.

The test code didn't set the u+x bit back to its original state
itself, instead relying on the subsequent call to git_config_set() to
re-write the config file with correct permissions.

But ever since

    daa22c6f8d config: preserve config file permissions on edits (2014-05-06)

git_config_set() copies the permissions from the old config file to
the new one. This is a good change in and of itself, but it interacts
badly with create_default_files()'s sloppiness, causing "git init" to
leave the executable bit set on $GIT_DIR/config.

So change create_default_files() to reset the permissions on
$GIT_DIR/config after its test.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
config: clear the executable bits (if any) on $GIT_DIR/config

There is no reason for $GIT_DIR/config to be executable, plus there
used to be a bug (fixed by the previous commit) that caused "git init"
to set the u+x bit on that file. So whenever rewriting the config
file, take the opportunity to remove any executable bits that the file
might have.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>