config: move flockfile() closer to unlocked functions

config: move flockfile() closer to unlocked functions

Commit 260d408e32 (config: use getc_unlocked when reading
from file, 2015-04-16) taught git_config_from_file() to lock
the filehandle so that we could safely use the faster
unlocked functions to access the handle.

However, it split the logic into two places:

  1. The master lock/unlock happens in git_config_from_file().

  2. The decision to use the unlocked functions happens in
     do_config_from_file().

That means that if anybody calls the latter function, they
will accidentally use the unlocked functions without holding
the lock. And indeed, git_config_from_stdin() does so.

In practice, this hasn't been a problem since this code
isn't generally multi-threaded (and even if some Git program
happened to have another thread running, it's unlikely to be
reading from stdin). But it's a good practice to make sure
we're always holding the lock before using the unlocked
functions.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>