Makefile: add xgettext target for *.sh files

Makefile: add xgettext target for *.sh files

Change the "pot" target to also extract strings from our $(SCRIPT_SH)
files with xgettext(1).

Note that due to Jonathan Nieder's trick of doing "mv $@+ $@" at the
end of the target the "pot" target will now warn:

    $ make pot
        XGETTEXT po/git.pot
    po/git.pot+: warning: Charset "CHARSET" is not a portable encoding name.
                          Message conversion to user's charset might not work.

This warnings is emitted because xgettext is writing into a non-*.pot
file, it's harmless however. The content that's written out is
equivalent to what it would be if we were writing directly into an
existing POT file with --join-existing.

As part of this change I've eliminated the && chain between xgettext
calls, this is incompatible with $(QUIET_XGETTEXT), if the && is left
in it'll emit:

    /bin/sh: @echo: not found

Since it's redundant (the Makefile will stop if there's an error) I've
removed it altogether.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>