present: don't drop commands that immediately follow text

Prior to this, the line identified as the start of a command by the test
for a leading dot was dropped. This behaviour is not documented in the
present package documentation.

Change-Id: Ib7354f0f4a661fccc65be5f92d487fe7f627302f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/260899
Run-TryBot: Ian Lance Taylor <iant@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cherry Zhang <cherryyz@google.com>

present/parse.go

@@ -525,6 +525,7 @@ func parseSections(ctx *Context, name, prefix string, lines *Lines, number []int
 					// Command breaks text block.
 					// Section heading breaks text block in markdown.
 					if text[0] == '.' || isSpeakerNote(text) {
+						lines.back()
 						break
 					}
 					if strings.HasPrefix(text, `\.`) { // Backslash escapes initial period.

present/testdata/media.p

@@ -2,6 +2,7 @@ Media
 
 *
 
+The Gopher
 .image gopher.jpg _ 100
 .caption A gopher.
 
@@ -14,6 +15,7 @@ Media
 ---
 <h1>Media</h1>
 <section>
+<p>The Gopher</p>
 <img src="gopher.jpg" width="100" alt="">
 <figcaption>A gopher.</figcaption>
 <iframe src="https://golang.org/"></iframe>