Introducing function for_all_git_repositories

Description
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    Find all Git repositories starting down from specified directory and
    execute specified command in each of those.

Usage
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    for_all_git_repositories DIRECTORY COMMAND [COMMAND_ARGUMENTS]

Examples
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    # Get all repos up-to-date.
    for_all_git_repositories ~/opt git pull

    # List all found repositories.
    for_all_git_repositories ~/opt pwd