git_handler: make refs a required param to getnewgitcommits

All its callers already specify this param.
git_handler: rename convert_list to commit_cache

convert_list is not the best naming choice for this variable. It is neither a
list nor has anything to directly do with conversion -- it is simply a cache of
commit hashes to the Git objects backing them.
git_handler: rename getnewgitcommits to get_git_incoming

getnewgitcommits isn't PEP8 and is kind of awkward -- what does 'new' mean?
git_handler.git_get_incoming: move default refs init to the top

This is preparation for moving most of the code into a separate module. These
two lines will be left behind in this module.
git_handler.get_git_incoming: explicitly use the Git object store

This is preparation for moving most of the code here out into a separate
module. With this patch, we can avoid passing in the entire Repo object and
just pass in the object store to the factored out function.
git_handler: move get_git_incoming to a separate module

git_handler's getting far too big, and we're going to add some more git-to-hg
specific logic soon, so now seems like a good time to add a new module.
git2hg: return a struct from find_incoming

We're going to return more data from find_incoming soon. Make that easier than
parsing a tuple.
git2hg.find_incoming: drop unnecessary if statement
git2hg.find_incoming: drop a comment and clarify another

The removed comment is irrelevant, and the second one confused me for a while.
git2hg.find_incoming: drop unused 'seen' set
git2hg.find_incoming: move head-finding code into a function

This is preparation for upcoming changes to find_incoming that will allow it to
import certain Git branches as Mercurial named branches.
git2hg.find_incoming: move graph traversal into a function

This is preparation for upcoming changes to find_incoming that will allow it to
import certain Git branches as Mercurial named branches.
git_handler: move extract_hg_metadata into git2hg

This function doesn't depend on self at all, so moving it to git2hg is
straightforward.