Use sigaction and SA_RESTART in read-tree.c; add option in Makefile.

Use sigaction and SA_RESTART in read-tree.c; add option in Makefile.

Might as well ape the sigaction change in read-tree.c to avoid
the same potential problems.  The fprintf status output will
be overwritten in a second, so don't bother guarding it.  Do
move the fputc after disabling SIGALRM to ensure we go to the
next line, though.

Also add a NO_SA_RESTART option in the Makefile in case someone
doesn't have SA_RESTART but does restart (maybe older HP/UX?).
We want the builder to chose this specifically in case the
system both lacks SA_RESTART and does not restart stdio calls;
a compat #define in git-compat-utils.h would silently allow
broken systems.

Signed-off-by: Jason Riedy <ejr@cs.berkeley.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>