histedit: add missing b prefix to a string

If i18n is disabled (such as via HGPLAIN=1), `_()` doesn't convert from str to
bytes, so this raises a TypeError on py3.

Differential Revision: https://phab.mercurial-scm.org/D8354

--HG--
branch : stable
10 jobs for 5.3.2 in 28 minutes and 58 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Test
passed #66453
checks-py2

00:02:29

passed #66454
checks-py3

00:03:52

passed #66455
rust-cargo-test-py2

00:02:55

passed #66456
rust-cargo-test-py3

00:01:32

passed #66457
test-py2

00:22:15

failed #66459
test-py2-pure

00:23:04

passed #66461
test-py2-rust

00:28:58

passed #66458
test-py3

00:21:05

passed #66460
test-py3-pure

00:19:55

passed #66462
test-py3-rust

00:20:42

 
Name Stage Failure
failed
test-py2-pure Test
Skipped test-sparse-fsmonitor.t: skipped
Skipped test-sparse-revlog.t: missing artifact, run "/tmp/mercurial-ci/tests/artifacts/scripts/generate-churning-bundle.py"
Skipped test-sqlitestore.t: missing feature: sqlite3 module is available
Skipped test-verify-repo-operations.py: missing feature: allow slow tests (use --allow-slow-tests)
Skipped test-wireproto-exchangev2-shallow.t: missing feature: sqlite3 module is available
Failed test-remotefilelog-prefetch.t: output changed
# Ran 850 tests, 56 skipped, 1 failed.
python hash seed: 3889787686
ERROR: Job failed: exit code 1