Ignore .hgtags in po2moz
Proper add and remove of files in Git VC
Introdice L10N_ENUS dir

This allows a portable way to reference the en-US templates dir.
Only update PO_DIR once

Since Git is a DVCS we only need to do this once.
Clean up the target pomigrate dir

Since in most casese the target is the same as the source, this ensures that
files no longer in the templates are not present after pomigrate'ing.
Ensure that POT files are also reflowed
Introduce retired dirs

This allows us to obsolete old files but not to clutter the current product
dirs with old ones.
Add newly arrived directories
Add FIXMEs re reflow points
Attempt to exlude retired products