Cast command to string in PhutilServiceProfiler

Summary: After the `PhutilCommandString` patch, DarkConsole/service profiler are showing `<object:PhutilCommandString>` instead of the actual command. String cast these so we get the command.

Test Plan: Viewed DarkConsole, saw a `git` commnand instead of `<object:PhutilCommandString>`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6595