pager: do not unnecessarily set COLUMNS on Windows

Since gwsw/less@bb0ee4e76c2, `less` prefers
the `COLUMNS` variable over asking ncurses itself.

This is typically not a problem because we ask `TIOCGWINSZ` in Git, to
determine the correct value for `COLUMNS`, and then set that environment
variable.

However, on Windows it _is_ a problem. The reason is that Git for
Windows uses a version of `less` that relies on the MSYS2 runtime to
interact with the pseudo terminal (typically inside a MinTTY window,
which is also aware of the MSYS2 runtime). Both MinTTY and `less.exe`
interact with that pseudo terminal via `ioctl()` calls (which the MSYS2
runtime emulates even if there is no such thing on Windows).

But `git.exe` itself is _not_ aware of the MSYS2 runtime, or for that
matter of that pseudo terminal, and has no way to call `ioctl()` or
`TIOCGWINSZ`.

Therefore, `git.exe` will fall back to hard-coding 80 columns, no matter
what the actual terminal size is.

But `less.exe` is totally able to interact with the MSYS2 runtime and
would not actually require Git's help (which actually makes things
worse here). So let's not override `COLUMNS` on Windows.

Note: we do this _only_ on Windows, and _only_ if `TIOCGWINSZ` is not
defined, to reduce any potential undesired fall-out from this patch.

This fixes #3235

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

pager.c

@@ -111,11 +111,13 @@ void setup_pager(void)
 	 * to get the terminal size. Let's grab it now, and then set $COLUMNS
 	 * to communicate it to any sub-processes.
 	 */
+#if !defined(WIN32) || defined(TIOCGWINSZ)
 	{
 		char buf[64];
 		xsnprintf(buf, sizeof(buf), "%d", term_columns());
 		setenv("COLUMNS", buf, 0);
 	}
+#endif
 
 	setenv("GIT_PAGER_IN_USE", "true", 1);