Do not use VISUAL editor on dumb terminals

Refuse to use $VISUAL and fall back to $EDITOR if TERM is unset
or set to "dumb".  Traditionally, VISUAL is set to a screen
editor and EDITOR to a line-based editor, which should be more
useful in that situation.

vim, for example, is happy to assume a terminal supports ANSI
sequences even if TERM is dumb (e.g., when running from a text
editor like Acme).  git already refuses to fall back to vi on a
dumb terminal if GIT_EDITOR, core.editor, VISUAL, and EDITOR are
unset, but without this patch, that check is suppressed by
VISUAL=vi.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/test-lib.sh b/t/test-lib.sh
index f2ca536..ec3336a 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -30,7 +30,7 @@
 TERM=dumb
 export LANG LC_ALL PAGER TERM TZ
 EDITOR=:
-VISUAL=:
+unset VISUAL
 unset GIT_EDITOR
 unset AUTHOR_DATE
 unset AUTHOR_EMAIL
@@ -58,7 +58,7 @@
 export GIT_MERGE_VERBOSITY
 export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
-export EDITOR VISUAL
+export EDITOR
 GIT_TEST_CMP=${GIT_TEST_CMP:-diff -u}
 
 # Protect ourselves from common misconfiguration to export
@@ -207,8 +207,8 @@
 test_set_editor () {
 	FAKE_EDITOR="$1"
 	export FAKE_EDITOR
-	VISUAL='"$FAKE_EDITOR"'
-	export VISUAL
+	EDITOR='"$FAKE_EDITOR"'
+	export EDITOR
 }
 
 test_tick () {