How do you include a Dev module in a make file?
You're almost there - what you need to add to your make file looks like this and you have to specify the git url and commit identifier.

projects[path_breadcrumbs][version] = "3.x-dev"
projects[path_breadcrumbs][download][type] = "git"
projects[path_breadcrumbs][download][url] = "http://git.drupal.org/project/path_breadcrumbs.git"
projects[path_breadcrumbs][download][revision] = "c9c570b0b4bde1df0e5a322ab90197d7fc1cf9c6"


To find this, go to the project page on drupal.org and select 'view commits', ex


On the next page, select the one you are interested in that reflects the state of the module you want to add to your make file. For this example, I'm going to grab the last commit: 



and I'm clicking the 'commit bdcf88a' link which takes me to: 

http://drupalcode.org/project/path_breadcrumbs.git/commit/bdccf8a

Now I know that the the [download][url] is:

http://drupalcode.org/project/path_breadcrumbs.git

and the full hash for the commit I am looking for is:

bdccf8a901b6d2359b6da195d23cadf7dd90c4ec

--

Further updates as requested -- if you're looking for a specific dev branch you may want to switch to the command line to do a --recursive clone to make sure you're getting all the branches. Following this question:

How do I git pull a dev branch?

I'm going to do a recursive pull of the git 7.x-1.x branch

git clone --recursive --branch 7.x-1.x http://git.drupal.org/project/fences.git


Now I can see what branches are available: 

git branch -r
  origin/1343724-region
  origin/7.x-1.x
  origin/HEAD -&gt; origin/master
  origin/master


Now I can checkout this branch for 7.x-1.x and check the log, which will give me the last commit and the one that was used to generate the development snapshot.

git checkout 7.x-1.x
git log

  commit 67206b5220cbebf52e8e6a290ccb977b2420b2ad
  Author: JohnAlbin &lt;virtually.johnalbin@gmail.com&gt;
  Date:   Tue Sep 4 13:44:40 2012 +0800

      Fix code comments on figcaption.


The date is different from what you may see on the project home page only because this is an automatic development snapshot created by the Drupal.org git bots but the development snapshot is always created from the branch with the same version number and the '-dev' part is never used in the branch name.