sendemail: teach git-send-email to dump alias names

sendemail: teach git-send-email to dump alias names

Add an option "--dump-aliases" which changes the default behavior of
git-send-email. This mode will simply read the alias files configured by
sendemail.aliasesfile and sendemail.aliasfiletype and dump a list of all
configured aliases, one per line. The intended use case for this option
is the bash-completion script which will use it to autocomplete aliases
on the options which take addresses.

Add some tests for the new option using various alias file formats.

A possible future extension to the alias dump format could be done by
extending the --dump-aliases to take an optional argument defining the
format to display. This has not been done in this patch as no user of
this information has been identified.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>