subtree: have $indent actually affect indentation

subtree: have $indent actually affect indentation

Currently, the $indent variable is just used to track how deeply we're
nested, and the debug log is indented by things like

   debug "  foo"

That is: The indentation-level is hard-coded.  It used to be that the
code couldn't recurse, so the indentation level could be known
statically, so it made sense to just hard-code it in the
output. However, since 315a84f9aa ("subtree: use commits before rejoins
for splits", 2018-09-28), it can now recurse, and the debug log is
misleading.

So fix that.  Indent according to $indent.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>