patchtest: test_shortlog_length: omit shortlog prefixes

patchtest: test_shortlog_length: omit shortlog prefixes

In certain cases, the shortlog for a patch would include branch and
'PATCH' tag information even after the repo tag got stripped out,
padding it with data that wouldn't actually be included with a merge.
This caused the patchtest test_shortlog_length test to erroneously
report a failure.

For example, a patch whose shortlog tags were:

[OE-core][master/scarthgap][PATCH]

would leave this behind in the shortlog, along with the actual content:

[master/scarthgap][PATCH]

Add a re.sub() call in the test to fix this by removing all square
bracket pairs that appear at the beginning of the string.

Fixes [YOCTO #15575].

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>