t6500: get hostname from "git var"

The test "background auto gc respects lock for all operations" simulates
a concurrent gc by writing our shell's pid and hostname to "gc.pid".
It gets the hostname from the "hostname" command, falling back to
"unknown" if the command fails.

"git gc" gets its hostname from the system and only checks whether the
pid is alive if the two hostnames match. Otherwise, it assumes the
lock belongs to a gc running on another machine. Since the test passes
in either case, a missing "hostname" command or a mismatch between
its output and the system hostname can cause the test to silently
exercise the wrong case.

Use "git var GIT_HOSTNAME" to get the same hostname as "git gc".
Keep the "unknown" fallback, which is also used by "git gc" when it
cannot determine the hostname.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Isabella Caselli <bellacaselli20@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6500-gc.sh

@@ -413,7 +413,7 @@ test_expect_success 'background auto gc respects lock for all operations' '
 
 	# now fake a concurrent gc that holds the lock; we can use our
 	# shell pid so that it looks valid.
-	hostname=$(hostname || echo unknown) &&
+	hostname=$(git var GIT_HOSTNAME || echo unknown) &&
 	shell_pid=$$ &&
 	if test_have_prereq MINGW && test -f /proc/$shell_pid/winpid
 	then