Add git_docs folder to depot_tools.

git_docs/git-freeze.txt

+git-freeze(1)
+=============
+
+NAME
+----
+git-freeze - Freeze all changes on a branch (indexed and unindexed)
+
+SYNOPSIS
+--------
+[verse]
+'git freeze'
+
+DESCRIPTION
+-----------
+
+`git freeze` works a lot like `git stash`, in that it stores the current changes
+in your working copy and index 'somewhere'. Unlike `git stash`, `git freeze`
+stores those changes on your current branch. This effectively allows you to
+'pause' development of a branch, work on something else, and then come back to
+exactly the same working state later (by running `git thaw`).
+
+`git freeze` will make up to 2 commits on your branch. A commit with the message
+`FREEZE.indexed` will contain all changes which you’ve added to your index (like
+with 'git add', 'git mv', 'git rm', etc.). A commit with the message
+`FREEZE.unindexed` will contain all changes which were not in your index at the
+time you ran git freeze (freshly modified files, new files, etc.).
+
+SEE ALSO
+--------
+linkgit:git-thaw[1]
+
+include::_footer.txt[]