git-gui: Don't require a .pvcsrc to create Tools/Migrate menu hack
The Tools/Migrate menu option is a hack just for me. Yes, that's right, git-gui has a hidden feature that really only works for me, and the users that I support within my day-job's great firewall. The menu option is not supported outside of that environment.
In the past we only enabled Tools/Migrate if our special local script 'gui-miga' existed in the proper location, and if there was a special '.pvcsrc' in the top level of the working directory. This latter test for the '.pvcsrc' file is now failing, as the file was removed from all Git repositories due to changes made to other tooling within the great firewall's realm.
I have changed the test to only work on Cygwin, and only if the special 'gui-miga' is present. This works around the configuration changes made recently within the great firewall's realm, but really this entire Tools/Migrate thing should be abstracted out into some sort of plugin system so other users can extend git-gui as they need.
Signed-off-by: Shawn O. Pearce <firstname.lastname@example.org>