git-gui: Guess our share/git-gui/lib path at runtime if possible

git-gui: Guess our share/git-gui/lib path at runtime if possible

Johannes Sixt asked me to try to avoid embedding the runtime location
of git-gui's library directory in the executable script.  Not embedding
it helps the MinGW to be relocatable to another directory should a user
wish to install the programs in a directory other than the location the
packager wanted them to be installed into.

Most of this is a hack.  We try to determine if the path of our master
git-gui script will be able to locate the lib by ../share/git-gui/lib.
This should be true if $(gitexecdir) and $(libdir) have the same prefix.
If they do then we defer the assignment of $(libdir) until runtime, and
we get it from $argv0 rather than embedding it into the script itself.

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