Support `branch` option

[keiranmraine]

When branch coverage is used the output isn't parsed by the headers, always assumes Stmnts, Miss, Cover.

When branch is used:

Name                  Stmts   Miss Branch BrPart     Cover
----------------------------------------------------------
src\rsm\__init__.py       0      0      0      0   100.00%
src\rsm\excel.py         80     11     28      5    85.19%
src\rsm\main.py          56      8     16      3    84.72%
src\rsm\utils.py         23      4      6      1    75.86%
----------------------------------------------------------
TOTAL                   159     23     50      9    83.73%

Required test coverage of 80.0% reached. Total coverage: 83.73%

But get:

Name	Stmts	Miss	Cover
/opt/hostedtoolcache/Python/3.10.5/x64/lib/python3.10/site-packages/rsm/init.py	0	0	0
/opt/hostedtoolcache/Python/3.10.5/x64/lib/python3.10/site-packages/rsm/excel.py	80	11	28
/opt/hostedtoolcache/Python/3.10.5/x64/lib/python3.10/site-packages/rsm/main.py	56	8	16
/opt/hostedtoolcache/Python/3.10.5/x64/lib/python3.10/site-packages/rsm/utils.py	23	4	6
TOTAL	159	23	50
Required	test	coverage	of

(final line relating to #98)

[mfitz]

Just to say, firstly, thanks for creating and open-sourcing this Github action, it's very useful, and secondly, supporting the --cov-branch flag would also be very helpful for me. We have a separate Github Actions workflow that does all of our CI checks, including a code coverage threshold check, and for that we generate the report with the pytest --cov-branch flag set.

Like @keiranmraine, I realised that using this flag breaks the table in the PR comment from pytest-coverage-commentator, so I have to leave that out when running the action. However, that means that the code coverage number in the comment disagrees with the one in our CI build, which causes some confusion:

With --cov-branch

Name                Stmts   Miss Branch BrPart  Cover
-----------------------------------------------------
aws_helper.py          35      0      2      0   100%
docker_helper.py       15      0      0      0   100%
matesto.py            123     22     28      2    79%
report_2_slack.py      64     11      8      1    81%
utils.py               54      0      0      0   100%
-----------------------------------------------------
TOTAL                 291     33     38      3    86%

Without --cov-branch

Name                Stmts   Miss  Cover
---------------------------------------
aws_helper.py          35      0   100%
docker_helper.py       15      0   100%
matesto.py            123     22    82%
report_2_slack.py      64     11    83%
utils.py               54      0   100%
---------------------------------------
TOTAL                 291     33    89%