summary message should not show ‘no changes’

[wbolster]

when direnv-mode is enabled, and direnv-always-show-summary is on, the summary message shows ‘no changes’ when changing between files within the same (direnv managed) project, when those files reside in different directories, e.g. implementation and tests.

it would be nice if the ‘no changes’ message was not shown.

[wbolster]

smarter default behaviour of summary messages on environment change

this changes the default behaviour to show a summary of environment
changes in the minibuffer, but only if there actually are changes.

for interactive calls, a summary message is always shown, as before.
explicit user interaction always leads to direct user feedback.

on the other hand, when using direnv-mode, and for non-interactive
calls (e.g. from hooks), the logic to decide whether to show a summary
message is now smarter than before. as a result the
direnv-always-show-summary variable is now enabled by default.

in practice, the new behaviour will avoid automatic useless minibuffer
messages (‘direnv: no changes’) all the time when switching between
files in different directories, but within the same direnv controlled
project directory, which is what happened before this change.

closes #23.