In Git, how could I search for a file or directory by path across a number of branches?

I've written something in a branch, but I don't remember which one. Now I need to find it.

Clarification: I'm looking for a file which I created on one of my branches. I'd like to find it by path, and not by its contents, as I don't remember what the contents are.

git log + git branch will find it for you:

% git log --all -- somefile
commit 55d2069a092e07c56a6b4d321509ba7620664c63
Author: Dustin Sallings <[email protected]>
Date:   Tue Dec 16 14:16:22 2008 -0800
added somefile
% git branch -a –contains 55d2069
otherbranch

Supports globbing, too:

% git log --all -- '**/my_file.png'

The single quotes are necessary (at least if using the Bash shell) so the shell passes the glob pattern to git unchanged, instead of expanding it (just like with Unix find).