Add a context menu command to load commits
that are within a certain time range from the
selected commit into gitk.

It can be useful for understanding of the code,
especially if the repository is imported from
a VCS that does not support atomic commits.
Signed-off-by: NAlexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>