Makefile: conditionally include GIT-VERSION-FILE

The 'clean' target is noticeably slow on cygwin, even for a 'do-nothing'
invocation of 'make clean'. For example, the second 'make clean' given
below:

  $ make clean >/dev/null 2>&1
  $ make clean
  GITGUI_VERSION = 0.21.0.85.g3e5c
  rm -rf git-gui lib/tclIndex po/*.msg
  rm -rf GIT-VERSION-FILE GIT-GUI-VARS
  $

has been timed at 1.934s on my laptop (an old core i5-4200M @ 2.50GHz,
8GB RAM, 1TB HDD).

Notice that the Makefile, as part of processing the 'clean' target, is
updating the 'GIT-VERSION-FILE' file.  This is to ensure that the
$(GITGUI_VERSION) make variable is set, once that file had been included.
However, the 'clean' target does not use the $(GITGUI_VERSION) variable,
so this 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 0.676s, on my laptop,
giving an improvement of 65.05%).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

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