git-gui: Automatically backup the user's commit buffer

git-gui: Automatically backup the user's commit buffer

A few users have been seeing crashes in Tk when using the undo key
binding to undo the last few keystroke events in the commit buffer.
Unfortunately that means the user loses their commit message and
must start over from scratch when the user restarts the process.

git-gui now saves the user's commit message buffer every couple of
seconds to a temporary file under .git (specifically .git/GITGUI_BCK).
At exit time we rename this file to .git/GITGUI_MSG if there is a
message, the file exists, and it is currently synchronized with the
Tk buffer.  Otherwise we do our usual routine of saving the Tk buffer
to .git/GITGUI_MSG and delete .git/GITGUI_BCK, if it exists.

During startup we favor .git/GITGUI_BCK over .git/GITGUI_MSG.  This
way a crash doesn't take out the user's message buffer but instead
will cause the user to lose only a few keystrokes.  Most people do
not type more than 200 WPM, and with 30 possible saves per minute
we are unlikely to lose more than 7 words.

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