Git: Show all of the various changes to a single line in a specified file over the entire git history

Since Git 1.8.4, there is a more direct way to answer your question.

Assuming that line 110 is the line saying var identifier = "SOME_IDENTIFIER";, then do this:

git log -L110,110:/lib/client.js

This will return every commit which touched that line of code.

See git-log‘s documentation for the -L command line parameter.