git-gui: offer a list of recent repositories on startup

git-gui: offer a list of recent repositories on startup

If git-gui is started outside a work tree the repository chooser
will offer a list of recently opened repositories.  Clicking on
any list entry directly opens the repository.

The list of recently opened repositories is stored in the config
as the multi-valued option gui.recentrepo. If the list grows beyond
10 entries it will be truncated by removing one of the older entries.

Only repositories that are opened through the repository chooser
will get added to the recent list.  Repositories opened from the
shell will not yet be added to the recent list, as users are likely
to have a way to easily return to the same directory via their shell.

[sp: This is actually a combined work from both Steffen and myself.
     Most of the ideas are Steffen's, as is the basic outline of
     the code, but any outstanding bugs are entirely my fault.]

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>