amtool: ignores custom path in url

[importantchoice]

What did you do?

Configured alertmanager to serve the webroot under /alerts.
Configured amtool to use this url:

alertmanager.url: "https://$USER:$PASSWORD@example.com/alerts"

What did you expect to see?

amtool alert query returns a list of current alerts

What did you see instead? Under which circumstances?

A 404 Error.
amtool: error: unknown error (status 404): {resp:0xc0005ae630}

This setup worked fine under version 0.15.2. After upgrading to version 0.17.0 the error above occures

Environment

• System information:

Linux 5.1.8-arch1-1-ARCH x86_64

• Alertmanager version:

alertmanager, version 0.17.0 (branch: HEAD, revision: c7551cd75c414dc81df027f691e2eb21d4fd85b2)
  build user:       root@932a86a52b76
  build date:       20190503-09:10:07
  go version:       go1.12.4

• amtool version:

amtool, version 0.17.0 (branch: HEAD, revision: c7551cd75c414dc81df027f691e2eb21d4fd85b2)
  build user:       root@932a86a52b76
  build date:       20190503-09:10:07
  go version:       go1.12.4

• Logs:

Nginx log:

XXX.XXX.XXX.XXX - - [24/Jun/2019:19:37:26 +0200] "GET /api/v2/alerts?active=true&inhibited=false&silenced=false&unprocessed=false HTTP/1.1" 404 136 "-" "Go-http-client/1.1"

As the /alerts path does not appear in the GET request, I guess amtool ignores the configured path and assumes that alertmanager is running directly under / for a given domain.