git-gui: fix error handling of Revert Changes command

The command Revert Changes has two different erroneous behaviors
depending on the Tcl version used.

The command uses a "chord" facility where different "notes" are
evaluated asynchronously and any error is reported after all of them
have finished. The intent is that a private namespace is used where
the notes can store the error state. Tcl 9 changed namespace handling
in a subtle way, as https://www.tcl-lang.org/software/tcltk/9.0.html
summarizes under "Notable incompatibilities":

Unqualified varnames resolved in current namespace, not global.
Note that in almost all cases where this causes a change, the
change is actually the removal of a latent bug.

And that's exactly what happens here.

- Under Tcl 9:

- When the command operates without any errors, the variable `err`
is never set. When the error handler wants to inspect `err` (in
the correct private namespace), it does not find it and a Tcl
error about an unset variable occurs. Incidentally, this is also
the case when the user cancels the operation with the option
"Do Nothing"!

On the other hand, when an error occurs during the operation, `err`
is set and found as intended.

Check for the existence of the variable `err` before the attempt to
read it.

- Under Tcl 8.6:

The error handler looks up `err` in the global namespace, which is
bogus and unintended. The variable is set due to the many
`catch ... err` that occur during startup in the global namespace.

- When the command operates without any errors, the error handler
finds the global `err`, which happens to be the empty string at
this point, and no error is reported.

On the other hand, when an error occurs during the operation, the
global `err` is set and found, so that an error is reported as
desired.

However, the value of `err` persists in the global namespace. When
the command is repeated, an error is reported again, even if there
was actually no error, and even "Do Nothing" was used to cancel
the operation.

Clear the global `err` before the operation begins.

The lingering error message is not a problem under Tcl 9, because a
prestine namespace is established every time the command is used.

This fixes https://github.com/j6t/git-gui/issues/21.

Helped-by: Igor Stepushchik
Signed-off-by: Johannes Sixt <j6t@kdbg.org>