Documentation/Makefile: conditionally include doc.dep

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

  $ make clean >/dev/null 2>&1
  $ make clean
  GIT_VERSION = 2.29.0
  ...
  make[1]: Entering directory '/home/ramsay/git/Documentation'
      GEN mergetools-list.made
      GEN cmd-list.made
      GEN doc.dep
  ...
  $

has been timed at 23.339s, using git v2.29.0, on my laptop (an old core
i5-4200M @ 2.50GHz, 8GB RAM, 1TB HDD).

Notice that, since the 'doc.dep' file does not exist, make takes the
time (about 8s) to generate several files in order to create the doc.dep
include file. (If an 'include' file is missing, but a target for the
said file is present in the Makefile, make will execute that target
and, if that file now exists, throw away all its internal data and
re-read and re-parse the Makefile). Having spent the time to include
the 'doc.dep' file, the 'clean' target immediately deletes those files.
The document dependencies specified in the 'doc.dep' include file,
expressed as make targets and prerequisites, do not affect what the
'clean' target removes. Therefore, the time spent in generating the
dependencies is completely wasted effort.

In order to eliminate such wasted effort, use the value of the internal
$(MAKECMDGOALS) variable to only '-include doc.dep' when the target is
not 'clean'. (This drops the time down to 12.364s, on my laptop, giving
an improvement of 47.02%).

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

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