allow TZ in _DATE metadata fields

[glensc]

simple implementation for #11

it currently just appends timezone info to GIT_COMMITTER_DATE, GIT_AUTHOR_DATE metadata values, so the change is not compatible if somebody modifies metadata date fields via FILTER_BRANCH_CALLBACK. however the conversion will abort with parse error, so it's not harmful.

if not using filter branch callback, no changes are visible, as +0000 timezone is used.

ideally to be compatible with git filter-branch should support all three Git date formats, but I haven't found good support in Python base libraries to do such parsing and Dulwich also doesn't have such methods.

[glensc]

here's sample FILTER_BRANCH_CALLBACK that converts dates to certain timezone (requires pytz and dateutil packages):

Before: '2015-04-11 15:34:24 +0000'
After: '2015-04-11 18:34:24 +0300'

import pytz
from dateutil.parser import parse

DATE_TIME_FORMAT = "%Y-%m-%d %H:%M:%S %z"
TZ = pytz.timezone('Europe/Tallinn')
def convert_localtime(timestr):
    localtime = parse(timestr).astimezone(TZ)
    return localtime.strftime(DATE_TIME_FORMAT)

def my_filter_callback(path, entry, metadata, treedir):
    # convert to local time
    metadata['GIT_AUTHOR_DATE'] = convert_localtime(metadata['GIT_AUTHOR_DATE'])
    metadata['GIT_COMMITTER_DATE'] = convert_localtime(metadata['GIT_COMMITTER_DATE'])

FILTER_BRANCH_CALLBACK = my_filter_callback

[glensc]

added the hook and example how to use it. looks good for merge!

[glensc]

@fragglet have got chance to give look at this?

[glensc]

@fragglet responded to current design decisions.

[glensc]

@fragglet https://github.com/fragglet/agito/pull/12#issuecomment-263023083 see my comments for reasons and say your final words. i don't need this otherwise, but would like to get it contributed back, someone might find it useful as i did.