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 <email@example.com>