i18n: config: unfold error messages marked for translation

Introduced in 473166b ("config: add 'origin_type' to config_source
struct", 2016-02-19), Git can inform the user about the origin of a
config error, but the implementation does not allow translators to
translate the keywords 'file', 'blob, 'standard input', and
'submodule-blob'. Moreover, for the second message, a reason for the
error is appended to the message, not allowing translators to translate
that reason either.

Unfold the message into several templates for each known origin_type.
That would result in better translation at the expense of code
verbosity.

Add enum config_oringin_type to ease management of the various
configuration origin types (blob, file, etc).  Previously origin type
was considered from command line if cf->origin_type == NULL, i.e.,
uninitialized. Now we set origin_type to CONFIG_ORIGIN_CMDLINE in
git_config_from_parameters() and configset_add_value().

For error message in git_parse_source(), use xstrfmt() function to
prepare the message string, instead of doing something like it's done
for die_bad_number(), because intelligibility and code conciseness are
improved for that instance.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule-config.c

@@ -448,7 +448,7 @@ static const struct submodule *config_from(struct submodule_cache *cache,
 	parameter.commit_sha1 = commit_sha1;
 	parameter.gitmodules_sha1 = sha1;
 	parameter.overwrite = 0;
-	git_config_from_mem(parse_config, "submodule-blob", rev.buf,
+	git_config_from_mem(parse_config, CONFIG_ORIGIN_SUBMODULE_BLOB, rev.buf,
 			config, config_size, &parameter);
 	free(config);