prepend path/to/phabricator/support/bin/ to $PATH

Summary:
It's sometimes necessary to specify the paths to individual binaries
explicitly, e.g. a particular build of 'javelinsymbols' or a newer
version of git than is installed on your shared system.

By adding symlinks in the .../phabricator/support/bin/ directory you
can now spell these out using the file system.

Test Plan:
Ran on local Ubuntu VM:

  .. add 'TEST' repo to diffusion ..
  .. visit 127.0.01/diffusion/TEST - see ok ..

  $ cd /opt
  $ sudo sh -c 'echo "exit 1" > badgit'
  $ sudo chmod +x /opt/badgit
  $ sudo mkdir goodgit
  $ sudo mv /usr/bin/git /opt/goodgit/

  .. unset environment.append-paths ..
  .. visit 127.0.01/diffusion/TEST - see error 'git: not found' ..
  .. set environment.append-paths to /opt/goodgit/ ..
  .. visit 127.0.01/diffusion/TEST - see ok ..

  $ sudo ln -s /opt/badgit /usr/bin/git
  .. visit 127.0.01/diffusion/TEST - see error 'error #1' ..
  sudo ln -s /opt/goodgit/git web/phabricator/support/bin/git
  .. visit 127.0.01/diffusion/TEST - see ok ..

  .. unset environment.append-paths ..
  .. visit 127.0.01/diffusion/TEST - see ok ..

  $ sudo rm web/phabricator/support/bin/git
  .. visit 127.0.01/diffusion/TEST - see error 'error #1' ..

  $ sudo rm /usr/bin/git
  $ sudo mv /opt/goodgit/git /usr/bin/
  .. visit 127.0.01/diffusion/TEST - see ok ..

Note that 'DIRECTORY_SEPARATOR' was not used because apparently it's
portable and ok to just use '/'.
http://alanhogan.com/tips/php/directory-separator-not-necessary
(I'm pretty new to PHP so looking for guidance :)

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2378

Differential Revision: https://secure.phabricator.com/D5561