Usage output should use `git foo` when invoked via git

If I run the git-cp script directly with no parameters, the output says USAGE: /foo/bar/bin/git-cp [...] which is on point. I might want it to just say git-cp there, but that's a minor quibble.

More importantly, and the subject of this issue, is that if I run git cp and git searches my path to find the git-cp script, then the output STILL says USAGE: /foo/bar/bin/git-cp [...] when I think it should say USAGE: git cp [...] in keeping with the behavior of most other git commands.

Some git command scripts included with git use different approaches for this:

• git-archimport, git-cvsexportcommit, git-cvsimport, git-cvsserver, and git-merge-one-file all hard code the usage text of git whatever without regard for the actual filename
• git-filter-branch, git-merge-one-file, git-mergetool, git-submodule, and git-web--browse all use git-sh-setup's usage() to parse the script filename and blindly removes the first dash
• git-p4 does what git-extras does now, outputting the full path to the script

I propose one of the following solutions:

1. Hard code git foo as the usage invocation for git-foo across all the scripts
2. Use git-sh-setup's usage()
3. Implement a custom detection of git invocation, perhaps by checking for GIT_EXEC_PATH

I volunteer to implement any of these if you pick one.

Start by comparing usage handling in the affected git-* scripts, especially git-archimport, git-filter-branch, git-mergetool, git-submodule, and git-web--browse, and read git-sh-setup's usage() behavior. Choose and apply a consistent approach for commands invoked as git foo so their usage output uses that invocation; direct script execution should remain understandable.