git-gui: Fix "unoptimized loading" to not cause git-gui to crash

git-gui: Fix "unoptimized loading" to not cause git-gui to crash

If the tclsh command was not available to us at the time we were
"built" our lib/tclIndex just lists all of our library files and
we source all of them at once during startup, rather than trying
to lazily load only the procedures we need.  This is a problem as
some of our library code now depends upon the git-version proc,
and that proc is not defined until after the library was fully
loaded.

I'm moving the library loading until after we have determined the
version of git we are talking to, as this ensures that the required
git-reversion procedure is defined before any library code can be
loaded.  Since error_popup is defined in the library we instead use
tk_messageBox directly for errors found during the version detection.

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