Makefile: sort source files before feeding to xgettext

Makefile: sort source files before feeding to xgettext

We will feed xgettext with more C source files and in different order
in subsequent commit. To generate a stable "po/git.pot" regardless of
the number and order of input source files, we sort the c, perl, and
shell source files in groups before feeding them to xgettext.

Ævar suggested that we should not pass the option "--sort-by-file" to
xgettext to sort the translatable strings, as it will mix the three
groups of source files (c, perl and shell) in the file "po/git.pot",
and change the order of translatable strings in the same line of a file.

With this update, the newly generated "po/git.pot" will have the same
entries while in a different order.

With the help of a custom diff driver as shown below,

    git config --global diff.gettext-fmt.textconv \
        "msgcat --no-location --sort-by-file"

and appending a new entry "*.pot diff=gettext-fmt" to git attributes,
we can see that there are no substantial changes in "po/git.pot".

We won't checkin the newly generated "po/git.pot", because we will
remove it from tree in a later commit.

Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>