GIT: How to calculate the contribution of each developer?

just the number of commits by user
git shortlog -s-n

It is believed that the quality of work of the programmer it is impossible to estimate the amount of code written, so there was a meme "the Hindu code" (progressive payment, etc)

Drew a simple gawk script which parses the results of the command 'git log --numstat'. In the end, the script gives this statistics for each developer (the author):
\r
Author: Scrooge McDuck
Insertions: 136
Deletions: 333
Files changed: 14
Commits: 15
\r
The script and details here: github.com/apis/gitstatistics

There is such a thing as Caelum git reports

Redmine makes such statistics for any VCS, which is connected to the project.

If we are talking about opencores, connect the repository to www.ohloh.net

gitstats.sourceforge.net/ makes the HTML there, and Qty. and by whom, and lines of code, and so on.