xgethostname: handle long hostnames

If the full hostname doesn't fit in the buffer supplied to
gethostname, POSIX does not specify whether the buffer will be
null-terminated, so to be safe, we should do it ourselves.  Introduce
new function, xgethostname, which ensures that there is always a \0
at the end of the buffer.

Signed-off-by: David Turner <dturner@twosigma.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/gc.c b/builtin/gc.c
index 5befd51..4f85610 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -232,7 +232,7 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 		/* already locked */
 		return NULL;
 
-	if (gethostname(my_host, sizeof(my_host)))
+	if (xgethostname(my_host, sizeof(my_host)))
 		xsnprintf(my_host, sizeof(my_host), "unknown");
 
 	pidfile_path = git_pathdup("gc.pid");