Fix fish script translation file generation

The previous version generates files which do not preserve the line number from
the original fish script file, resulting in translation not working.

The new approach is quite ugly, and might have some issues,
but at least it seems to work in some cases.

build_tools/fish_xgettext.fish

@@ -39,8 +39,28 @@ end >messages.pot
 function extract_fish_script_messages --argument-names name regex;
     mkdir -p $tmpdir/$name/share/completions $tmpdir/$name/share/functions
     for f in share/config.fish share/completions/*.fish share/functions/*.fish
-        string replace --filter --regex $regex '$1' <$f | string unescape \
-            | string replace --all '"' '\\"' | string replace -r '(.*)' 'N_ "$1"' >$tmpdir/$name/$f
+        # Extract messages from fish script.
+        # This is done by creating a file which has the message strings in the same lines as the
+        # oritinal, in the form:
+        # N_ "message"
+        # All other lines will be empty.
+        # Multiple messages on a single line are not supported.
+
+        # Start by transforming the matching lines according to $explicit_regex and adding 'N_ ' as
+        # a prefix.
+        # Then, replace all lines without this prefix by empty lines.
+        # These lines are kept to ensure that the correct line number makes it into the pot file.
+        # Replacing irrelevant lines by empty lines must happen before unescaping, because otherwise
+        # multi-line commands (lines ending on \) would get merged, changing the line count.
+        # Double quotes are escaped, and finally unescaped double quotes are added around the string.
+        # The result will be a file consisting of empty lines and potentially some lines prefixed with
+        # 'N_ ', followed by a double-quoted string.
+        string replace --regex $regex 'N_ $1' <$f |
+            sed '/^N_ / !{s/^.*$//}' |
+            string unescape |
+            string replace --all '"' '\\"' |
+            string replace --regex '^N_ (.*)$' 'N_ "$1"' \
+            >$tmpdir/$name/$f
     end
 end