git grep <pattern> will search only one commit.
If you want to search all of git's history, use one of the pickaxe options :
git log -G "specific_text" -- frequently/modified/file # or : git log -S "specific_text" -- frequently/modified/file
The difference between the two is explained in the docs :
-G will list all commits where specific_text appears in the diff, -S will only list commits where the count of specific_text changes, e.g : the commit where this text appears, the commit where it gets deleted, but not a commit which just moves it around, without adding or removing an instance.
You can also add -p if you want to see the diff itself :
git log -p -S "specific_text" -- frequently/modified/file
find, it is calledgrep. Typegit help grepin your terminal or read its documentation on the Git website.