fast-import: do less work when given "from" matches current branch head

When building a fast-import stream, it's easy to forget the fact
that for non-merge commits happening on top of the current branch
head, there is no need for a "from" command. That is corroborated by
the fact that at least git-p4, hg-fast-export and felipec's
git-remote-hg all unconditionally use a "from" command.

Unfortunately, giving a "from" command always resets the branch
tree, forcing it to be re-read, and in many cases, the pack is also
closed and reopened through gfi_unpack_entry.  Both are unnecessary
overhead, and the latter is particularly slow at least on OSX.

Avoid resetting the tree when it's unmodified, and avoid calling
gfi_unpack_entry when the given mark points to the same commit as
the current branch head.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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