doc build: speed up "make lint-docs"

doc build: speed up "make lint-docs"

Extend the trick we use to speed up the "clean" target to also extend
to the "lint-docs" target. See 54df87555b1 (Documentation/Makefile:
conditionally include doc.dep, 2020-12-08) for the "clean"
implementation.

The "doc-lint" target only depends on *.txt files, so we don't need to
generate GIT-VERSION-FILE etc. if that's all we're doing. This makes
the "make lint-docs" target more than 2x as fast:

$ git show HEAD~:Documentation/Makefile >Makefile.old
$ hyperfine -L f ",.old" 'make -f Makefile{f} lint-docs'
Benchmark #1: make -f Makefile lint-docs
  Time (mean ± σ):     100.2 ms ±   1.3 ms    [User: 93.7 ms, System: 6.7 ms]
  Range (min … max):    98.4 ms … 103.1 ms    29 runs

Benchmark #2: make -f Makefile.old lint-docs
  Time (mean ± σ):     220.0 ms ±  20.0 ms    [User: 206.0 ms, System: 18.0 ms]
  Range (min … max):   206.6 ms … 267.5 ms    11 runs

Summary
  'make -f Makefile lint-docs' ran
    2.19 ± 0.20 times faster than 'make -f Makefile.old lint-docs'

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>