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