I keep a directory with up-to-date clones of all relevant repos. This is separate from my usual working directory. I experimented with git workspaces, but it wasn't worth the trouble, especially since the set of repos I'm working on is not necessarily the same as the ones I keep in my search dir.

At the root level I maintain two scripts:

  clone.sh

  update.sh

clone.sh has one

  git clone --recursive .. 

line per repo. When I run low on disk space I sometimes delete larger repos. The clone script allows me to easily re-clone everything in this case.

update.sh is similar but pulls all repos.

For global search across all branches I do:

  git grep <regexp> $(git rev-list --all)

(when I forget the line I look it up in Stack Overflow [1])

This is especially useful since I work a lot with Bitbucket and to the best of my knowledge you can only search the default branches there.

When I know the branch, but want to search across all history I use git pickaxe, aka

git -S ...

All of this is not very sophisticated and takes a lot of disk space but it works pretty well for me.

[1] https://stackoverflow.com/a/15293283