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 <email@example.com>