git log -c -S'missingtext' /path/to/file
git log doesn’t show a diff for merge commits by default. Try the -c or --cc flags.
More discussion/explanation:
https://git-scm.com/docs/git-log
nabble.com
From the git-log docs:
-c With this option, diff output for a merge commit shows the differences from each of the parents to the merge result
simultaneously instead of showing pairwise diff between a parent and
the result one at a time. Furthermore, it lists only files which were
modified from all parents.–cc This flag implies the -c option and further compresses the patch output by omitting uninteresting hunks whose contents in the
parents have only two variants and the merge result picks one of them
without modification.