从另一个 git 存储库链接单个文件

Considering that the unit of work for git is a repository (or more precisely a repository content), I don't think you can easily integrate one file.

If you don't need its history, you could consider simply copy it in your repo.
But if you do need the history, then some git filter-branch (as in "git: How to split off library from project? filter-branch, subtree?") are in order. That seems a lot of effort for just one file though.

In theory, "git: symlink/reference to a file in an external repository" suggests a solution combining submodule and symlink.
(from Pavel Šimerda)

$ git submodule add /url/submodule/<reponame>
$ ln -s <reponame>/path/to/<linked_file>
$ git add .gitmodules <linked_file>
$ git commit -m "add a symbolic link to <linked_file> with the respective submodule"

Since 2011 (original answer above), Gavriel reports in the comments that:

• When I checkout the main repo, the linked file is not a symlink, but instead the file content is "<reponame>/path/to/<linked_file>". In other words git commits the symlink's target as file content
• But the basic idea works.
  I use "submodule/path/to/file" instead of "file" and it works