i18n: treat "make pot" as an explicitly-invoked target

i18n: treat "make pot" as an explicitly-invoked target

po/git.pot is normally used as-is and not regenerated by people
building git, so it is okay if an explicit "make po/git.pot" always
automatically regenerates it.  Depend on the magic FORCE target
instead of explicitly keeping track of dependencies.

This simplifies the makefile, in particular preparing for a moment
when $(LIB_H), which is part of $(LOCALIZED_C), can be computed on the
fly. It also fixes a slight breakage in which changes to perl and shell
scripts did not trigger a rebuild of po/git.pot.

We still need a dependency on GENERATED_H, to force those files to be
built when regenerating git.pot.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>