The CL introducing merged handling of cover profiles
did not correctly account for the fact that the file name argument
to -coverprofile is required to be interpreted relative to
the -outputdir argument.
Fixes #22804.
Change-Id: I804774013c12187313b8fd2044302978bdbb6697
Reviewed-on: https://go-review.googlesource.com/81455
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>