git-gui: Allow digging through history in blame viewer

gitweb has long had a feature where the user can click on any
commit the blame display and go visit that commit's information
page.  From the user could go get the blame display for the file
they are tracking, and try to digg through the history of any
part of the code they are interested in seeing.

We now offer somewhat similiar functionality in git-gui.  The 4
digit commit abreviation in the first column of our blame view is
now offered as a hyperlink if the commit isn't the one we are now
viewing the blame output for (as there is no point in linking back
to yourself).  Clicking on that link will stop the current blame
engine (if still running), push the new target commit onto the
history stack, and restart the blame viewer at that commit, using
the "original file name" as supplied by git-blame for that chunk
of the output.

Users can navigate back to a version they had been viewing before
by way of a back button, which offers the prior commits in a popup
menu displayed right below the back button.  I'm always showing the
menu here as the cost of switching between views is very high; you
don't want to jump to a commit you are not interested in looking at
again.

During switches we throw away all data except the cached commit data,
as that is relatively small compared to most source files and their
annotation marks.  Unfortunately throwing this per-file data away in
Tcl seems to take some time; I probably should move the line indexed
arrays to proper lists and use [lindex] rather than the array lookup
(usually lists are faster).

We now start the git-blame process using "nice", so that its priority
will drop hopefully below our own.  If I don't do this the blame engine
gets a lot of CPU under Windows 2000 and the git-gui user interface is
almost non-responsive, even though Tcl is just sitting there waiting
for events.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

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