Resolve "Should we git stash pop after a gdk update?"

This MR advises the user if we've performed a git stash for a given component (gitlab, gitaly, etc):

$ make update
(in /Users/ash/src/gdk/gdk-primary)
ok: run: ./services/postgresql: (pid 36193) 4446s, normally down
ok: run: ./services/postgresql-geo: (pid 36194) 4446s, normally down
ok: run: ./services/praefect: (pid 36195) 4446s, normally down
ok: run: ./services/praefect-gitaly-0: (pid 36197) 4446s, normally down
ok: run: ./services/redis: (pid 36196) 4446s, normally down

-------------------------------------------------------
Updating gitlab to current master..
-------------------------------------------------------

~~~~~~~~~~~~~~~~~~~~~~~ WARNING ~~~~~~~~~~~~~~~~~~~~~~~
We're stashing the following changes with `git stash`:

 README.md | 1 +
 1 file changed, 1 insertion(+)

To restore these changes, run `git stash pop`.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Already on 'master'
Your branch is up to date with 'origin/master'.

-------------------------------------------------------
Updating gitlab-shell to v10.3.0..
-------------------------------------------------------

~~~~~~~~~~~~~~~~~~~~~~~ WARNING ~~~~~~~~~~~~~~~~~~~~~~~
We're stashing the following changes with `git stash`:

 README.md | 1 +
 1 file changed, 1 insertion(+)

To restore these changes, run `git stash pop`.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fetching origin
HEAD is now at 24156ea Merge branch 'release-10-3-0' into 'master'

-------------------------------------------------------
Updating gitlab-workhorse to v8.18.0..
-------------------------------------------------------
Fetching origin
HEAD is now at 9a9a83e Merge branch 'release-8-18' into 'master'

Closes #745 (closed)