path/filepath: HasPrefix should be marked deprecated

[dsnet]

The documentation for filepath.HasPrefix says:

HasPrefix exists for historical compatibility and should not be used.

We should switch this to use the "Deprecated:" tag so that tools can flag new uses of this.

As it is currently implemented, it is misleading because it only does strings.HasPrefix and does not actually match on patch segments. Thus, the following reports true, which is surprising:

filepath.HasPrefix("/home/userwrongdir/abc", "/home/user") // This reports true

\cc @neild

[bradfitz]

Just send it now for Go 1.8. It's only docs.

[gopherbot]

CL https://golang.org/cl/34554 mentions this issue.

[dsnet]

Opened #18358 to investigate actually fixing this function.

[RobbieMcKinstry]

Just checking: this function is deprecated, and there is no alternative? Maybe a regex.Match? I assume that's why there's interest in fixing.

[bradfitz]

@RobbieMcKinstry, your comment would be better on an open bug, like #18358. Closed bugs aren't tracked.

[RobbieMcKinstry]

Thank you Brad! Still learning! :)