oe-build-perf-test: new {tag_num} keyword for --commit-results-tag

oe-build-perf-test: new {tag_num} keyword for --commit-results-tag

This makes it possible to create numbered tags, where the "basename" of
the tag is the same and the only difference is an (automatically)
increasing index number. This is useful if you do multiple test runs on
the same commit. For example, using:
--commit-results-tag {tester_host}/{git_commit}/{tag_num}

would give you tags something like:
myhost/decb3119dffd3fd38b800bebc1e510f9217a152e/0
myhost/decb3119dffd3fd38b800bebc1e510f9217a152e/1
...

The default tag format is updated to use this new keyword in order to
prevent unintentional tag name clashes.

(From OE-Core rev: cf2aba16338a147f81802f48d2e24a96c7133548)

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>