git-gui: Rephrase rescan before commit informational message.

git-gui: Rephrase rescan before commit informational message.

Its not an error that a rescan is required before commit; its just
something we do as a safety feature to try and ensure the user knows
what is going into this commit.  So the dialog should use the info
icon (if one is used by the host OS) rather than the error icon.

Its also not "highly likely" that another Git program modified the
repository, its completely the case.  There is no reason why the
repository would not match our last scanned state unless another
Git program modified the repository (or someone else did so by hand).
So don't be vague about it, own up to the issue and go on with our
business.

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