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

src/infrastructure/env/PhabricatorEnv.php

···
       date_default_timezone_set('UTC');
     }
 
-    // Append any paths to $PATH if we need to.
-    $paths = PhabricatorEnv::getEnvConfig('environment.append-paths');
-    if (!empty($paths)) {
-      $current_env_path = getenv('PATH');
-      $new_env_paths = implode(PATH_SEPARATOR, $paths);
-      putenv('PATH='.$current_env_path.PATH_SEPARATOR.$new_env_paths);
+    // Prepend '/support/bin' and append any paths to $PATH if we need to.
+    $env_path = getenv('PATH');
+    $phabricator_path = dirname(phutil_get_library_root('phabricator'));
+    $support_path = $phabricator_path.'/support/bin';
+    $env_path = $support_path.PATH_SEPARATOR.$env_path;
+    $append_dirs = PhabricatorEnv::getEnvConfig('environment.append-paths');
+    if (!empty($append_dirs)) {
+      $append_path = implode(PATH_SEPARATOR, $append_dirs);
+      $env_path = $env_path.PATH_SEPARATOR.$append_path;
     }
+    putenv('PATH='.$env_path);
 
     PhabricatorEventEngine::initialize();