Documentation/Makefile: conditionally include ../GIT-VERSION-FILE

The 'clean' target is still noticeably slow on cygwin, despite the
substantial improvement made by the previous patch. For example, the
second invocation of 'make clean' below:

  $ make clean >/dev/null 2>&1
  $ make clean
  ...
  make[1]: Entering directory '/home/ramsay/git/Documentation'
  make[2]: Entering directory '/home/ramsay/git'
  make[2]: 'GIT-VERSION-FILE' is up to date.
  make[2]: Leaving directory '/home/ramsay/git'
  ...
  $

has been timed at 12.364s on my laptop (an old core i5-4200M @ 2.50GHz,
8GB RAM, 1TB HDD).

Notice that the 'clean' target is making a nested call to the parent
Makefile to ensure that the GIT-VERSION-FILE is up-to-date (prior to
the previous patch, there would have been _two_ such invocations).
This is to ensure that the $(GIT_VERSION) make variable is set, once
that file had been included.  However, the 'clean' target does not use
the $(GIT_VERSION) variable, directly or indirectly, so it does not
have any affect on what the target removes. Therefore, the time spent
on ensuring an up to date GIT-VERSION-FILE is wasted effort.

In order to eliminate such wasted effort, use the value of the internal
$(MAKECMDGOALS) variable to only '-include ../GIT-VERSION-FILE' when the
target is not 'clean'. (This drops the time down to 10.361s, on my laptop,
giving an improvement of 16.20%).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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