Shared: `makeSections` behavior changes.

+ When the optional base level parameter is provided, we no
  longer ensure that the sequence of heading levels is gapless
  [behavior change]. Instead, we set the lowest heading level to
  the specified base level, and adjust the others accordingly. If
  an author wants to skip a level, e.g. from level 1 to level 3,
  they can do that. In general, the heading levels specified
  in the source document are preserved; `makeSections` only
  puts them into a hierarchical structure. Closes #9398.

+ Section numbers are now assigned so that the top level
  gets `1`, no matter what heading level is used. So, even
  if the top heading level is 2, numbers will be `1`, `2`, etc.
  rather than `0.1`, `0.2`, as in the past. Closes #5071.

+ We revert to the old behavior when the `--number-offset` option
  is used. So, for example, if a document begins with a level-3
  heading, and `--number-offset=1,2` is used, the top-level section
  numbers will be `1.2.1`, `1.2.2`, etc. This is mainly for
  backwards-compatibility.

1 files changed, 68 insertions, 0 deletions

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+```
+% pandoc -f markdown -t html --number-sections
+## First section
+
+### Subhead
+
+##### Subhead with gap
+
+## Second section
+^D
+<h2 data-number="1" id="first-section"><span
+class="header-section-number">1</span> First section</h2>
+<h3 data-number="1.1" id="subhead"><span
+class="header-section-number">1.1</span> Subhead</h3>
+<h5 data-number="1.1.0.1" id="subhead-with-gap"><span
+class="header-section-number">1.1.0.1</span> Subhead with gap</h5>
+<h2 data-number="2" id="second-section"><span
+class="header-section-number">2</span> Second section</h2>
+
+```
+
+```
+% pandoc -f markdown -t html --number-sections
+## First section
+
+### Subhead
+
+# Higher-level section
+
+## Sub
+^D
+<h2 data-number="0.1" id="first-section"><span
+class="header-section-number">0.1</span> First section</h2>
+<h3 data-number="0.1.1" id="subhead"><span
+class="header-section-number">0.1.1</span> Subhead</h3>
+<h1 data-number="1" id="higher-level-section"><span
+class="header-section-number">1</span> Higher-level section</h1>
+<h2 data-number="1.1" id="sub"><span
+class="header-section-number">1.1</span> Sub</h2>
+```
+
+For backwards compatibility, we want it to work the old way,
+giving numbers like 0.1, when `--number-offset` is used:
+```
+% pandoc -f markdown -t html --number-sections --number-offset=2,2,2
+## First section
+
+### Subhead
+^D
+<h2 data-number="2.3" id="first-section"><span
+class="header-section-number">2.3</span> First section</h2>
+<h3 data-number="2.3.3" id="subhead"><span
+class="header-section-number">2.3.3</span> Subhead</h3>
+
+```
+
+```
+% pandoc -f markdown -t html --number-sections --number-offset=0,2,2
+## First section
+
+### Subhead
+^D
+<h2 data-number="0.3" id="first-section"><span
+class="header-section-number">0.3</span> First section</h2>
+<h3 data-number="0.3.3" id="subhead"><span
+class="header-section-number">0.3.3</span> Subhead</h3>
+
+```