git-gui: Improve directions regarding POT update in po/README

Keeping POT up to date relative to the software is absolutely
necessary.  What is unwarranted is updating language files at
the same time by running msgmerge without checking if there is
any outstanding translation work first.  If we assume that the
translators do not have access to msgmerge, that is a good service
to them (the less they have to do, the better), but otherwise,
it is better to be leave po/${language}.po files alone.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

po/README

@@ -178,18 +178,6 @@ step.
 
 	$ msgmerge -U po/af.po po/git-gui.pot
 
-[NEEDSWORK: who is responsible for updating po/git-gui.pot file by
-running xgettext?  IIRC, Christian recommended against running it
-nilly-willy because it can become a source of unnecessary merge
-conflicts.  Perhaps we should mention something like "
-
-The po/git-gui.pot file is updated by the internationalization
-coordinator from time to time.  You _could_ update it yourself, but
-translators are discouraged from doing so because we would want all
-language teams to be working off of the same version of git-gui.pot.
-
-" here?]
-
 This updates po/af.po (again, replace "af" with your language
 code) so that it contains msgid lines (i.e. the original) that
 your translation did not have before.  There are a few things to
@@ -207,3 +195,53 @@ watch out for:
 
  - New messages added to the software will have msgstr lines with empty
    strings.  You would need to translate them.
+
+The po/git-gui.pot file is updated by the internationalization
+coordinator from time to time.  You _could_ update it yourself, but
+translators are discouraged from doing so because we would want all
+language teams to be working off of the same version of git-gui.pot.
+
+****************************************************************
+
+This section is a note to the internationalization coordinator, and
+translators do not have to worry about it too much.
+
+The message template file po/git-gui.pot needs to be kept up to date
+relative to the software the translations apply to, and it is the
+responsibility of the internationalization coordinator.
+
+When updating po/git-gui.pot file, however, _never_ run "msgmerge -U
+po/xx.po" for individual language translations, unless you are absolutely
+sure that there is no outstanding work on translation for language xx.
+Doing so will create unnecessary merge conflicts and force needless
+re-translation on translators.  The translator however may not have access
+to the msgmerge tool, in which case the coordinator may run it for the
+translator as a service.
+
+But mistakes do happen.  Suppose a translation was based on an older
+version X, the POT file was updated at version Y and then msgmerge was run
+at version Z for the language, and the translator sent in a patch based on
+version X:
+
+         ? translated
+        /
+    ---X---Y---Z (master)
+
+The coordinator could recover from such a mistake by first applying the
+patch to X, replace the translated file in Z, and then running msgmerge
+again based on the updated POT file and commit the result.  The sequence
+would look like this:
+
+    $ git checkout X
+    $ git am -s xx.patch
+    $ git checkout master
+    $ git checkout HEAD@{1} po/xx.po
+    $ msgmerge -U po/xx.po po/git-gui.pot
+    $ git commit -c HEAD@{1} po/xx.po
+
+State in the message that the translated messages are based on a slightly
+older version, and msgmerge was run to incorporate changes to message
+templates from the updated POT file.  The result needs to be further
+translated, but at least the messages that were updated by the patch that
+were not changed by the POT update will survive the process and do not
+need to be re-translated.