git-gui: Consolidate hook execution code into a single function

git-gui: Consolidate hook execution code into a single function

The code we use to test if a hook is executable or not differs on
Cygwin from the normal POSIX case.  Rather then repeating that for
all three hooks we call in our commit code path we can place the
common logic into a global procedure and invoke it when necessary.

This also lets us get rid of the ugly "|& cat" we were using before
as we can now rely on the Tcl 8.4 feature of "2>@1" or fallback to
the "|& cat" when necessary.

The post-commit hook is now run through the same API, but its outcome
does not influence the commit status.  As a result we now show any of
the errors from the post-commit hook in a dialog window, instead of on
the user's tty that was used to launch git-gui.  This resolves a long
standing bug related to not getting errors out of the post-commit hook
when launched under git-gui.

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