Add more FileSystemWatcher patterns, to improve refresh of submodules

[goran-w]

Fixes #1344.

Added a few more patterns in Watcher.OnRepositoryChanged() and Watcher.OnWorkingCopyChanged() :

• Skip files frequently updated by Git fsmonitor--daemon and Visual Studio, to ease debugging and for early exit.
• Skip .git file under submodule directories, for consistency.
• Check also for ORIG_HEAD under .git/modules/<submodule>/, to improve handling of submodules.
• Check for MERGE_HEAD and AUTO_MERGE under .git/, to improve handling of submodules (etc) during merging.

NOTE: Maybe the MERGE_HEAD and/or AUTO_MERGE files should affect some other update-timers as well, if needed to improve refresh during Merging? (Currently only affecting _updateSubmodules and _updateWC.)