interpret-trailers: tighten check for "---" patch boundary

The interpret-trailers command accepts not only raw commit
messages, but it also can manipulate trailers in
format-patch output. That means it must find the "---"
boundary separating the commit message from the patch.
However, it does so by looking for any line starting with
"---", regardless of whether there is further content.

This is overly lax compared to the parsing done in
mailinfo.c's patchbreak(), and may cause false positives
(e.g., t/perf output tables uses dashes; if you cut and
paste them into your commit message, it fools the parser).

We could try to reuse patchbreak() here, but it actually has
several heuristics that are not of interest to us (e.g.,
matching "diff -" without a three-dash separator or even a
CVS "Index:" line). We're not interested in taking in
whatever random cruft people may send, but rather handling
git-formatted patches.

Note that the existing documentation was written in a loose
way, so technically we are changing the behavior from what
it said. But this should implement the original intent in a
more accurate way.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-interpret-trailers.txt

@@ -56,8 +56,9 @@ least one Git-generated or user-configured trailer and consists of at
 least 25% trailers.
 The group must be preceded by one or more empty (or whitespace-only) lines.
 The group must either be at the end of the message or be the last
-non-whitespace lines before a line that starts with '---'. Such three
-minus signs start the patch part of the message.
+non-whitespace lines before a line that starts with '---' (followed by a
+space or the end of the line). Such three minus signs start the patch
+part of the message.
 
 When reading trailers, there can be whitespaces after the
 token, the separator and the value. There can also be whitespaces

t/t7513-interpret-trailers.sh

@@ -1417,4 +1417,26 @@ test_expect_success 'unfold' '
 	test_cmp expected actual
 '
 
+test_expect_success 'handling of --- lines in input' '
+	echo "real-trailer: just right" >expected &&
+
+	git interpret-trailers --parse >actual <<-\EOF &&
+	subject
+
+	body
+
+	not-a-trailer: too soon
+	------ this is just a line in the commit message with a bunch of
+	------ dashes; it does not have any syntactic meaning.
+
+	real-trailer: just right
+	---
+	below the dashed line may be a patch, etc.
+
+	not-a-trailer: too late
+	EOF
+
+	test_cmp expected actual
+'
+
 test_done

trailer.c

@@ -793,7 +793,9 @@ static size_t find_patch_start(const char *str)
 	const char *s;
 
 	for (s = str; *s; s = next_line(s)) {
-		if (starts_with(s, "---"))
+		const char *v;
+
+		if (skip_prefix(s, "---", &v) && isspace(*v))
 			return s - str;
 	}