am: Fix launching of pager

The pagination functionality in git am has some problems:

 - It does not check if stdout is a tty, so it always paginates.

 - If $GIT_PAGER uses any environment variables, they are being
   ignored, since it does not run $GIT_PAGER through eval.

 - If $GIT_PAGER is set to the empty string, instead of passing
   output through to stdout, it tries to run $dotest/patch.

Fix them.  While at it, move the definition of git_pager() to
git-sh-setup so authors of other commands are not tempted to
reimplement it with the same mistakes.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 14 insertions, 4 deletions

diff --git a/git-am.sh b/git-am.sh
index 3c08d53161..b11af03e0b 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -663,10 +663,7 @@ do
 		[eE]*) git_editor "$dotest/final-commit"
 		       action=again ;;
 		[vV]*) action=again
-		       : ${GIT_PAGER=$(git var GIT_PAGER)}
-		       : ${LESS=-FRSX}
-		       export LESS
-		       $GIT_PAGER "$dotest/patch" ;;
+		       git_pager "$dotest/patch" ;;
 		*)     action=again ;;
 		esac
 	    done
diff --git a/git-sh-setup.sh b/git-sh-setup.sh
index d56426dd39..44fb4670ae 100755
--- a/git-sh-setup.sh
+++ b/git-sh-setup.sh
@@ -107,6 +107,19 @@ git_editor() {
 	eval "$GIT_EDITOR" '"$@"'
 }
 
+git_pager() {
+	if test -t 1
+	then
+		GIT_PAGER=$(git var GIT_PAGER)
+	else
+		GIT_PAGER=cat
+	fi
+	: ${LESS=-FRSX}
+	export LESS
+
+	eval "$GIT_PAGER" '"$@"'
+}
+
 sane_grep () {
 	GREP_OPTIONS= LC_ALL=C grep "$@"
 }