my.coding.commits: more cleanup

Followup of https://github.com/karlicoss/HPI/pull/132

- add REQUIRES section
- use 'commits' config section & add proper schema
- use dedicated subdirectory for cache

tox.ini

@@ -34,7 +34,7 @@ commands =
     # my.body.weight dep
     hpi module install my.orgmode
 
-    pip install gitpython # my.commits dep
+    hpi module install my.coding.commits
 
     python3 -m pytest tests \
             # ignore some tests which might take a while to run on ci..
@@ -79,7 +79,7 @@ commands =
     hpi module install my.stackexchange.stexport
     hpi module install my.pinboard
     hpi module install my.arbtt
-    pip install gitpython # my.commits dep
+    hpi module install my.coding.commits
 
     # todo fuck. -p my.github isn't checking the subpackages?? wtf...
     # guess it wants .pyi file??