git-gui: Allow the user to disable update-index --refresh during rescan.

git-gui: Allow the user to disable update-index --refresh during rescan.

On very large projects (~1000 files) on Windows systems the update-index
--refresh stage of the rescan process takes a nontrival amount of time.
If the user is generally very careful with their file modification such
that the modification timestamp of the file differs only when the content
also differs then we can skip this somewhat expensive step and go right
to the diff-index and diff-files processes.

We save the user's prefernce in the current repository if they modify the
setting during a git-gui session, but we also load it through our dump of
repo-config --list so the user could move it to their ~/.gitconfig file
if they wanted it globally disabled.

We still keep update-index --refresh enabled by default however, as most
users will probably want it.

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