git-gui: Jump to the first annotation block as soon as its available.

git-gui: Jump to the first annotation block as soon as its available.

To help clue users into the fact that annotation data arrives
incrementally, and that they should try to locate the region
they want while the tool is running, we jump to the first line
of the first annotation if the user has not already clicked on
a line they are interested in and if the window is still looking
at the very top of the file.

Since it takes a second (at least on my PowerBook) to even generate
the first annotation for git-gui.sh, the user should have plenty of
time to adjust the scrollbar or click on a line even before we get
that first annotation record in, which allows the user to bypass
our automatic jumping.

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