pager: remove 'S' from $LESS by default

By default, Git used to set $LESS to -FRSX if $LESS was not set by
the user. The FRX flags actually make sense for Git (F and X because
sometimes the output Git pipes to less is short, and R because Git
pipes colored output). The S flag (chop long lines), on the other
hand, is not related to Git and is a matter of user preference. Git
should not decide for the user to change LESS's default.

More specifically, the S flag harms users who review untrusted code
within a pager, since a patch looking like:

    -old code;
    +new good code; [... lots of tabs ...] malicious code;

would appear identical to:

    -old code;
    +new good code;

Users who prefer the old behavior can still set the $LESS environment
variable to -FRSX explicitly, or set core.pager to 'less -S'.

The documentation in config.txt is made a bit longer to keep both an
example setting the 'S' flag (needed to recover the old behavior)
and an example showing how to unset a flag set by Git.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -558,14 +558,19 @@ core.pager::
 	configuration, then `$PAGER`, and then the default chosen at
 	compile time (usually 'less').
 +
-When the `LESS` environment variable is unset, Git sets it to `FRSX`
+When the `LESS` environment variable is unset, Git sets it to `FRX`
 (if `LESS` environment variable is set, Git does not change it at
 all).  If you want to selectively override Git's default setting
-for `LESS`, you can set `core.pager` to e.g. `less -+S`.  This will
+for `LESS`, you can set `core.pager` to e.g. `less -S`.  This will
 be passed to the shell by Git, which will translate the final
-command to `LESS=FRSX less -+S`. The environment tells the command
+command to `LESS=FRX less -S`. The environment does not set the
-to set the `S` option to chop long lines but the command line
+`S` option but the command line does, instructing less to truncate
-resets it to the default to fold long lines.
+long lines. Similarly, setting `core.pager` to `less -+F` will
+deactivate the `F` option specified by the environment from the
+command-line, deactivating the "quit if one screen" behavior of
+`less`.  One can specifically activate some flags for particular
+commands: for example, setting `pager.blame` to `less -S` enables
+line truncation only for `git blame`.
 +
 Likewise, when the `LV` environment variable is unset, Git sets it
 to `-c`.  You can override this setting by exporting `LV` with

git-sh-setup.sh

@@ -160,7 +160,7 @@ git_pager() {
 	else
 		GIT_PAGER=cat
 	fi
-	: ${LESS=-FRSX}
+	: ${LESS=-FRX}
 	: ${LV=-c}
 	export LESS LV
 

pager.c

@@ -85,7 +85,7 @@ void setup_pager(void)
 		int i = 0;
 
 		if (!getenv("LESS"))
-			env[i++] = "LESS=FRSX";
+			env[i++] = "LESS=FRX";
 		if (!getenv("LV"))
 			env[i++] = "LV=-c";
 		env[i] = NULL;

perl/Git/SVN/Log.pm

@@ -116,7 +116,7 @@ sub run_pager {
 		return;
 	}
 	open STDIN, '<&', $rfd or fatal "Can't redirect stdin: $!";
-	$ENV{LESS} ||= 'FRSX';
+	$ENV{LESS} ||= 'FRX';
 	$ENV{LV} ||= '-c';
 	exec $pager or fatal "Can't run pager: $! ($pager)";
 }