perf doc: Set man page date to last git commit

perf doc: Set man page date to last git commit

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.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>