Auto-deactivate venvs in $PROJECT_HOME

This is an attempt to improve my previous pull request about making the
auto-activation plugin project aware (justinmayer#175). The solution back then was
to only deactivate virtualenvs of projects automatically if they contain
a .project file. However, this solution feels very unintuitive to me,
partly because it seems against the 'intended' use of the projects
plugin, which suggests to only use .project files if the project is not
in the $PROJECT_HOME directory - and I like to keep all my projects in
$PROJECT_HOME.

This patch aims to accommodate this workflow by auto-deactivating
virtualenvs of projects in $PROJECT_HOME without needing .project or
.venv files. It adds a check to the auto-activation plugin:
auto-deactivate venv if a) it's not auto-activated and doesn't contain a
.project file, AND b) it's not a subdirectory of $PROJECT_HOME. By
testing for these very specific conditions, this patch should not
interfere with the normal usage of .venv and .project files in any way.

virtualfish/auto_activation.fish

@@ -14,8 +14,10 @@ function __vfsupport_auto_activate --on-variable PWD
     set -l activation_root $PWD
     set -l new_virtualenv_name ""
 
-    # Projects plugin compatibility: Enable auto-deactivation (1/3)
-    # Check if active virtualenv (if any) is connected to a project
+    # Projects plugin compatibility: Enable auto-deactivation (1/4)
+    # Activating projects that don't contain activation files doesn't trigger VF_AUTO_ACTIVATED.
+    # To detect projects and determine when to auto-deactivate their virtualenv in those cases,
+    # check for .project files (and $PROJECT_HOME below).
     if test -e "$VIRTUAL_ENV/.project"
         set project_path (command cat "$VIRTUAL_ENV/.project")
     end
@@ -25,10 +27,9 @@ function __vfsupport_auto_activate --on-variable PWD
             set new_virtualenv_name (command cat "$activation_root/$VIRTUALFISH_ACTIVATION_FILE")
             break
 
-        # Projects plugin compatibility: Enable auto-deactivation (2/3)
-        # vf workon might activate virtualenvs without activation files. To detect those instances
-        # here in the Auto-activation plugin, check if activation root is a project path. If so,
-        # set new_virtualenv_name to the basename of the project path
+        # Projects plugin compatibility: Enable auto-deactivation (2/4)
+        # Check if activation root is a project path defined by a .project file. If so, set
+        # new_virtualenv_name to the basename of the project path
         else if test "$project_path" = "$activation_root"
             set new_virtualenv_name (command basename $project_path)
             break
@@ -45,14 +46,21 @@ function __vfsupport_auto_activate --on-variable PWD
             vf activate $new_virtualenv_name
             set -g VF_AUTO_ACTIVATED $activation_root
         end
+    # Projects plugin compatibility: Enable auto-deactivation (3/4)
+    # Projects stored in $PROJECT_HOME typically don't have .project files. To accommodate users
+    # that keep all their projects in $PROJECT_HOME, deactivate virtualenv if it wasn't
+    # auto-activated, doesn't contain .project file and $PWD is not a sudirectory of $PROJECT_HOME
+    else if begin set -q VIRTUAL_ENV; and not set -q VF_AUTO_ACTIVATED project_path; end
+        if begin not string match -qr -- "$PROJECT_HOME" "$PWD"; or test "$PROJECT_HOME" = "$PWD"; end
+            vf deactivate
+        end
     else
         # if there's an auto-activated virtualenv, deactivate it
         if set -q VIRTUAL_ENV VF_AUTO_ACTIVATED
             vf deactivate
 
-        # Projects plugin compatibility: Enable auto-deactivation (3/3)
-        # vf workon doesn't set VF_AUTO_ACTIVATED. To deactivate virtualenv automatically when
-        # leaving project directory, we check if any virtualenv is active while not in project path
+        # Projects plugin compatibility: Enable auto-deactivation (4/4)
+        # Deactivate project virtualenv when not in path specified in .project
         else if begin set -q VIRTUAL_ENV; and test "$project_path" != "$activation_root"; end
             vf deactivate
         end