git-p4: fix error handling in P4Unshelve.renameBranch()

git-p4: fix error handling in P4Unshelve.renameBranch()

The error handling code path is meant to be triggered when the loop does
not exit early via "break". This fails, as the boolean variable "found",
which is used to track whether the loop was exited early, is initialized
incorrectly.

It would be possible to fix this issue by correcting the initialization,
but Python supports a for:-else: control flow construct for this exact
use case (executing code if a loop does not exit early), so it is more
idiomatic to remove the tracking variable entirely.

In addition, the error message no longer refers to a variable that does
not exist.

Signed-off-by: Moritz Baumann <moritz.baumann@sap.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>