i18n: bisect: simplify error message for i18n

The message was not being extracted by xgettext, although it was marked
for translation, seemingly because it contained a command substitution.
Moreover, eval_gettext should be used instead of gettext for strings
with substitution.

See step 4. of section 15.5.2.1 Preparing Shell Scripts for
Internationalization from gettext manual [1]:
"Simplify translatable strings so that they don't contain command
substitution ("`...`" or "$(...)") [...]"

[1] http://www.gnu.org/software/gettext/manual/html_node/Preparing-Shell-Scripts.html

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

git-bisect.sh

@@ -277,8 +277,9 @@ bisect_state() {
 	0,*)
 		die "$(gettext "Please call 'bisect_state' with at least one argument.")" ;;
 	1,"$TERM_BAD"|1,"$TERM_GOOD"|1,skip)
-		rev=$(git rev-parse --verify $(bisect_head)) ||
-			die "$(gettext "Bad rev input: $(bisect_head)")"
+		bisected_head=$(bisect_head)
+		rev=$(git rev-parse --verify "$bisected_head") ||
+			die "$(eval_gettext "Bad rev input: \$bisected_head")"
 		bisect_write "$state" "$rev"
 		check_expected_revs "$rev" ;;
 	2,"$TERM_BAD"|*,"$TERM_GOOD"|*,skip)