git-gui (Windows): Change wrapper to execdir 'libexec/git-core'

git-gui (Windows): Change wrapper to execdir 'libexec/git-core'

git-gui needs bindir in PATH to be able to run 'git'.  bindir
however is not necessarily in PATH if started directly through a
Windows shortcut.  Therefore, we used to add the directory
git-gui is located in.  But with the new 'libexec/git-core'
layout this directory is no longer identical to bindir.

This commit modifies the wrapper script to discover the bindir
and add it to PATH.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>