Add git_docs folder to depot_tools.

git_docs/git-thaw.txt

Index: git_docs/git-thaw.txt
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+git-thaw(1)
+=============
+
+NAME
+----
+git-thaw - Un-freeze all changes on a frozen branch.
+
+SYNOPSIS
+--------
+[verse]
+'git thaw'
+
+DESCRIPTION
+-----------
+
+`git thaw` takes a frozen branch (as made by linkgit:git-freeze[1]), and returns
+it to the status that it was in before it was frozen.
+
+If you freeze a branch multiple times (i.e. change files, freeze, change files,
+freeze), `git thaw` will thaw all of the freezes.
+
+SEE ALSO
+--------
+linkgit:git-freeze[1]
+
+include::_footer.txt[]