git: ensure 0/1/2 are open in main()

git: ensure 0/1/2 are open in main()

Not having an open FD in the 0--2 range can lead to strange results,
for example, a subsequent open() may return 2 (stderr) and then a
die() would clobber this file.

git-daemon and git-shell already guarded against this, but apparently
users also manage to trip over it in other git commands.  So we call
sanitize_stdfds() during main git startup.

Since these FDs are inherited, this covers all use of 'git foo ...',
and all internal C commands when called directly.  It does not fix
shell/perl commands called directly.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>