fast-import: align some error messages with git branch

We do not write "GIT" in all caps anymore, and we do translate error
messages.

The error messages when a ref needs to be created and it fails either
because the ref name is invalid or because the ref already exists should
be identical to the ones `git branch` emits.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/fast-import.c

@@ -613,9 +613,9 @@ static struct branch *new_branch(const char *name)
 	struct branch *b = lookup_branch(name);
 
 	if (b)
-		die("Invalid attempt to create duplicate branch: %s", name);
+		die(_("a branch named '%s' already exists"), name);
 	if (check_refname_format(name, REFNAME_ALLOW_ONELEVEL))
-		die("Branch name doesn't conform to GIT standards: %s", name);
+		die(_("'%s' is not a valid branch name"), name);
 
 	b = mem_pool_calloc(&fi_mem_pool, 1, sizeof(struct branch));
 	b->name = mem_pool_strdup(&fi_mem_pool, name);