restore gitpython needed for upload

[tfoote]

Truly the last step failed https://build.ros.org/job/doc_rosindex/1851/

18:33:17 
18:33:17  
18:33:17                     done in 14882.704 seconds.
18:33:17  Auto-regeneration: disabled. Use --watch to enable.
18:33:17 make: Leaving directory '/tmp/doc_repository'
18:33:18 Traceback (most recent call last):
18:33:18   File "/usr/local/bin/git_add_files", line 4, in <module>
18:33:18     import git
18:33:18 ModuleNotFoundError: No module named 'git'
18:33:18 Build step 'Execute shell' marked build as failure
18:33:19 Build did not succeed and the project is configured to only push after a successful build, so no pushing will occur.
18:33:19 Finished: FAILURE

[tfoote]

Self merging to get the upload working again.