Move .git_cl_description_backup to depot_tools dir

It was previously stored in each repository working dir, making it
neccessary to modify .gitignore files in many places. Save it in
depot_tools working directory, where there's already an entry in its
.gitignore file and no additional work is needed to hide this file.

Bug: None
Change-Id: I58102180e37358a206c8ecd1044aa772a684e066
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1944167
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -138,7 +138,7 @@ def DieWithError(message, change_desc=None):
 
 
 def SaveDescriptionBackup(change_desc):
-  backup_path = os.path.join(settings.GetRoot(), DESCRIPTION_BACKUP_FILE)
+  backup_path = os.path.join(DEPOT_TOOLS, DESCRIPTION_BACKUP_FILE)
   print('\nsaving CL description to %s\n' % backup_path)
   backup_file = open(backup_path, 'w')
   backup_file.write(change_desc.description)