Create `overlap_days` variable for commits

[aaron-lohmann]

As per mail of Gabor create a variable capturing the overlap in commit days of users.

For each repo * coder 1, coder 2 (where coder 1 is the coder with the first commit), we could calculate
collab_total_days = number of days between the first commit by coder 1 and last commit by either coder.
collab_overlap_days = number of days that overlap (HERE I tried to teach chatgpt make a SQL for it)
collab_is_concurrent =0/1. =1 if collab_overlap_days/collab_total_days>collab_concurrent_threshold ( say =25%)