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

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,
which we know always exists, 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
invalidates the create_default_files()'s assumption, causing "git
init" to leave the executable bit set on $GIT_DIR/config.

Reset the permissions on $GIT_DIR/config when we are done with the
test in create_default_files().

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>