upload: Allow relative between different uploaders

[jerry-skydio]

When originally adding Uploader, we didn't allow relative between
different uploaders to be safe / prevent risk of mistakes. It seems
like there's a decently large use case for this though, so switch to
allowing it.

Topic: relup
Uploader: who
Reviewers: aaron, brian-k

[jerry-skydio]

Reviews in this chain:
└#205 upload: Allow relative between different uploaders

[jerry-skydio]

#	head	base	diff	date	summary
0	4a14bbd9	c7b92e83	diff	Dec 28 16:54 PM	1 file changed, 10 deletions(-)
1	bdffc6d6	c7b92e83	diff	Dec 28 16:57 PM	0 files changed
2	5d4048f9	c7b92e83	diff	Dec 28 17:27 PM	1 file changed, 1 insertion(+)
3	bdffc6d6	c7b92e83	diff	Dec 28 17:31 PM	1 file changed, 1 deletion(-)

[aaron-skydio]

Nice, yeah this will definitely get some use. Can we add an example of how to use this somewhere, maybe in the README?