t: strip bashism from test scripts

sh does not like identifiers with hyphens in them; replace them with
underscores, and change the she-bang from /bin/bash to /bin/sh.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
multimail: simplify GitObject by replacing .commit

The commit attribute of a GitObject object serves the sole purpose of
conveying the SHA-1 of the underlying commit if the object is a tag.
Replace it with a commit_sha1 member to reduce the overhead.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Merge branch 'simplify-gitobject'

Replace member GitObject.commit with lighter-weight
GitObject.commit_sha1.
test-emails: Set USER to make the results more repeatable.

The tests use GenericEnvironment, which uses $USER to determine the
pusher.  So set USER in the environment before running the
email-generation commands to make the results independent of the user
who happens to be running the tests.

Suggested by: Ramkumar Ramachandra <artagnon@gmail.com>