releasenotes: use re.MULTILINE mode when checking admonitions

Release note admonitions must start at the beginning of a line within
the changeset description:

.. admonitions::

The checkadmonitions function search for and validates admonitions.

Unfortunately, since the ctx.description is multi-line, the regex search
always fails unless the admonition is on the first line.

This changeset adds re.MULTILINE to the re.compile to make the re opbject
multi-line.

hgext/releasenotes.py

@@ -70,7 +70,7 b' DEFAULT_SECTIONS = ['
     (b'api', _(b'API Changes')),
 ]
 
-RE_DIRECTIVE = re.compile(br'^\.\. ([a-zA-Z0-9_]+)::\s*([^$]+)?$')
+RE_DIRECTIVE = re.compile(br'^\.\. ([a-zA-Z0-9_]+)::\s*([^$]+)?$', re.MULTILINE)
 RE_ISSUE = br'\bissue ?[0-9]{4,6}(?![0-9])\b'
 
 BULLET_SECTION = _(b'Other Changes')

tests/test-releasenotes-formatting.t

@@ -387,6 +387,8 b' Testing output for the --check (-c) flag'
 
   $ touch a
   $ hg -q commit -A -l - << EOF
+  > commit 2
+  > 
   > .. asf::
   > 
   >    First paragraph under this admonition.
@@ -395,7 +397,7 b' Testing output for the --check (-c) flag'
 Suggest similar admonition in place of the invalid one.
 
   $ hg releasenotes -r . -c
-  Invalid admonition 'asf' present in changeset 4026fe9e1c20
+  Invalid admonition 'asf' present in changeset 99fa3c800c5e
 
   $ touch b
   $ hg -q commit -A -l - << EOF
@@ -405,7 +407,7 b' Suggest similar admonition in place of t'
   > EOF
 
   $ hg releasenotes -r . -c
-  Invalid admonition 'fixes' present in changeset 0e7130d2705c
+  Invalid admonition 'fixes' present in changeset 4737b1b5afd1
   (did you mean fix?)
 
   $ cd ..