Make git version numbers external to batch script.

bootstrap/win/README.md

 # Windows binary tool bootstrap
 
 This directory has the 'magic' for the `depot_tools` windows binary update
 mechanisms.
 
 ## Software bootstrapped
   * Python (https://www.python.org/)
   * Git for Windows (https://git-for-windows.github.io/)
   * Subversion (https://subversion.apache.org/)
 
@@ skipping 20 matching lines @@
   * Original: `https://github.com/git-for-windows/git/releases`
   * Mirror: `gs://chrome-infra/PortableGit*.7z.exe`
 
 #### Updating git version
   1. Download the new `PortableGit-X.Y.Z-{32,64}.7z.exe` from the
      git-for-windows release page.
   1. From either console.developers.google.com or the CLI, do:
     1. Upload those to the gs://chrome-infra Google Storage bucket.
     1. Set the `allUsers Reader` permission (click the "Public link" checkbox
        next to the binaries).
-  1. Edit the `set GIT_VERSION=X.Y.Z` line in `win_tools.bat` to be the new
-     version.
-    1. At the time of writing, the first version is the default version, and
-       the second is the 'bleeding edge' version. You can use the bleeding edge
-       version to get early feedback/stage a rollout/etc.
+  1. Edit the `git_version.txt` or `git_version_bleeding_edge.txt` file to
+     be the new version.
+    1. You can use the bleeding edge version to get early feedback/stage a
+       rollout/etc. Users can select this version by 'touch'ing the
+       `.git_bleeding_edge` file in the root depot_tools directory.
   1. Commit the CL.
 
 Note that in order for the update to take effect, `gclient` currently needs to
 run twice. The first time it will update the `depot_tools` repo, and the second
 time it will see the new git version and update to it. This is a bug that should
 be fixed, in case you're reading this and this paragraph infuriates you more
 than the rest of this README.
 
 ### Python
 
 Python installs are sourced from gs://chrome-infra/python276_bin.zip .
 
 The process to create them is sort-of-documented in the README of the python
 zip file.
 
 ### Subversion
 
 Subversion installs are sourced from gs://chrome-infra/svn_bin.zip .
 
 There will likely never be an update to SVN in `depot_tools` from the current
 version.