git-gui: add directory git-gui is located in to PATH (on Windows)

git-gui: add directory git-gui is located in to PATH (on Windows)

This commit modifies PATH to include a good guess where git
could be found. The first location to search for executable is
the directory git-gui is installed in. This is a good guess for
a sane installation.

Even if git is not available in PATH, git-gui is now able
to find it. Hence git-gui can be passed to wish as an absolute
path without caring about the environment.

We must modify PATH to be able to spawn shell based git commands.
For builtins it would be sufficient to located them and execute
them with their absolute path. But for shell based git commmands
PATH needs to be modified.

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