mail.sh: various minor updates

mail.sh: various minor updates

Show the diff for the first revision on a new branch.

Improve rename detection in diffs (add the -B option).

Show a summary of changes for new branches just like
updated branches get.

Include a stat and summary for each individual diff that's
not a merge commit.

Limit the stat width to 72 for better email compatibility.

Show diffs for "evil" merges (add the --cc option).

Abbreviate merge commit hashes a bit to avoid overflowing
email lines so much.

Avoid error message if .git/description is not present.

Send output to stdout instead of mailing with --stdout as
the first argument.

Update a few comments to match the actual behavior.

Show dates using rfc2822 format since output is a mail message.

Fully peel tag objects when handling them.

Include embedded tag name in tag emails.

Produce more concise URLs and move them into the commit header area.

Add URLs for non-branch create/update events.

Force rev-parse to interpret its arguments as revisions.

Support hooks.summaryonly boolean to suppress actual diff itself.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>