scripts: add mailmapper, a tool to create/update mailmap file

scripts: add mailmapper, a tool to create/update mailmap file

This tool helps to create/update the mailmap file.

It runs 'git shortlog' internally and searches differently spelled author
names which share the same email address. The author name with the most
commits is asuumed to be a canonical real name. If the number of commits
from the cananonical name is equal to or greater than 'MIN_COMMITS' (=50),
the entry for the cananical name will be output. ('MIN_COMMITS' is used
here because we do not want to create a fat mailmap by adding every author
with only a few commits.)

If there exists a mailmap file specified by the mailmap.file configuration
options or '.mailmap' at the toplevel of the repository, it is used as
a base file.

The base file and the newly added entries are merged together and sorted
alphabetically (but the comment block is kept untouched), and then printed
to standard output.

 Usage
 -----

  scripts/mailmapper

prints the mailmapping to standard output.

  scripts/mailmapper > tmp; mv tmp .mailmap

will be useful for updating '.mailmap' file.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>