Currently the man page dates reflect the date the man pages were built.
This patch adjusts the date so that the date is when then man page
last had a commit against it. The date is generated using 'git log'.

Committer testing:

  $ git log -1 --pretty="format:%cd" --date=short tools/perf/Documentation/perf-top.txt
  2020-01-14

Before:

  rm -rf /tmp/build/perf
  mkdir -p /tmp/build/perf
  make -C tools/perf O=/tmp/build/perf/ install
  $ date
  Wed 11 Mar 2020 10:21:19 AM -03
  $ man perf-top | tail -1
  perf                    03/11/2020           PERF-TOP(1)
  $

After:

  rm -rf /tmp/build/perf
  mkdir -p /tmp/build/perf
  make -C tools/perf O=/tmp/build/perf/ install
  $ date
  $ date
  Wed 11 Mar 2020 10:24:06 AM -03
  $ man perf-top | tail -1
  perf                    2020-01-14           PERF-TOP(1)
  $
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masanari Iida <standby24x7@gmail.com>
Cc: Mukesh Ojha <mojha@codeaurora.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lore.kernel.org/lkml/20200311052110.23132-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>