If the command is either `diff` or `log`, there is often a lot of
lines of output. Run these commands through $GIT_PAGER, $PAGER, or
`less` in order to make it easier to browse the output on a terminal.
Change-Id: I18b87ea4acf404b94788f2ac2101812bd13e6a0f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>