If I’m looking for code changes:

    git log -S <the-string>

If it’s commit messages I’m interested in:

    git log --grep <the-word-or-phrase>

Sometimes I want to know “all the commits that introduced a pattern”:

    git blame —- $(git grep -P -l 'the.*regex') | grep -P 'the.*regex'

This last one I use frequently enough that I wrote a little shell script for it, called "git-blep" (for "blame-grep")

———

If there’s a function name that you’re logging, then you might get mileage from

   git log -L :<the-function-name>:<the-file-name>

Though that probably would not have worked in the original author’s case.