filter-repo: use our own textdomain for translations

git.git wants to move more towards core-only rather than batteries
included, and as such, filter-repo will not be part of the git
distribution.  Therefore, due to keeping the projects apart, there will
need to be separate translation files (assuming filter-repo ever gains
any translations) and as such we will need a different textdomain
definition.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -61,7 +61,7 @@ def gettext_poison(msg):
 _ = gettext_poison
 
 def setup_gettext():
-  TEXTDOMAIN="git"
+  TEXTDOMAIN="git-filter-repo"
   podir = os.environ.get("GIT_TEXTDOMAINDIR") or "@@LOCALEDIR@@"
   if not os.path.isdir(podir): # pragma: no cover
     podir = None  # Python has its own fallback; use that