Bring MANIFEST.in up to date.
When talking about Git in general, captialize "Git".

This is the current practice within the Git project itself.
t/README: Improve text and update description of files.

* Move the "Short instructions" higher in the file.

* Describe the new scripts.

* Add a section "Other files", describing the non-executable files in
  the directory.
Rename t/test-emails to t/generate-test-emails.

By itself, it isn't actually a test.
Rename t/compare-emails to t/test-email-content.

Make it clearer that this is a test script.
README: Various textual tweaks.
README: Adapt for planned submission to Git project.

Explain that, although git-multimail might be distributed with Git, it
is still developed as a separate project, and give instructions for
finding and getting involved with the git-multimail project.
Ignore file t/env-repo.git.

This directory is used to hold the test repository used by t/test-env.
It should be deleted automatically when test-env is done, but just in
case it is not, add it to .gitignore to prevent it from being
committed accidentally.
Merge branch 'doc-updates'

* Various documentation updates, including:

  * Document testing files better

  * Preparation for renewed submission to the Git project

* Rename some files in the testing directory for clarity.