The 'v'iew subcommand of the interactive mode of "git am -i" was
broken by the rewrite to C we did at around 2.6.0 timeframe at
7ff26832 (builtin-am: implement -i/--interactive, 2015-08-04); we
used to spawn the pager via the shell, accepting things like

	PAGER='less -S'

in the environment, but the rewrite forgot and tried to directly
spawn a command whose name is the entire string.

The previous refactoring of the new helper function makes it easier
for us to do the right thing.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>