x/tools/go/vcs: FromDir doesn't work with srcRoot = "/"

[hectorj]

_, _, err := vcs.FromDir("/whatever/", "/")

gives the error directory "/whatever" is outside source root "/", which is incorrect

[ianlancetaylor]

CC @paranoiacblack

[hectorj]

https://go-review.googlesource.com/#/c/13422/

[dmitshur]

Thanks for the report. This package has been deprecated in #57051 and won't receive further development, so closing this as not planned.