userdiff: support Markdown

It's typical to find Markdown documentation alongside source code, and
having better context for documentation changes is useful; see also
commit 69f9c87d4 (userdiff: add support for Fountain documents,
2015-07-21).

The pattern is based on the CommonMark specification 0.29, section 4.2
<https://spec.commonmark.org/> but doesn't match empty headings, as
seeing them in a hunk header is unlikely to be useful.

Only ATX headings are supported, as detecting setext headings would
require printing the line before a pattern matches, or matching a
multiline pattern. The word-diff pattern is the same as the pattern for
HTML, because many Markdown parsers accept inline HTML.

Signed-off-by: Ash Holland <ash@sorrel.sh>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -824,6 +824,8 @@ patterns are available:
 
 - `java` suitable for source code in the Java language.
 
+- `markdown` suitable for Markdown documents.
+
 - `matlab` suitable for source code in the MATLAB and Octave languages.
 
 - `objc` suitable for source code in the Objective-C language.

t/t4018-diff-funcname.sh

@@ -38,6 +38,7 @@ diffpatterns="
 	golang
 	html
 	java
+	markdown
 	matlab
 	objc
 	pascal

t/t4018/markdown-heading-indented

@@ -0,0 +1,6 @@
+Indented headings are allowed, as long as the indent is no more than 3 spaces.
+
+   ### RIGHT
+
+- something
+- ChangeMe

t/t4018/markdown-heading-non-headings

@@ -0,0 +1,17 @@
+Headings can be right next to other lines of the file:
+# RIGHT
+Indents of four or more spaces make a code block:
+
+    # code comment, not heading
+
+If there's no space after the final hash, it's not a heading:
+
+#hashtag
+
+Sequences of more than 6 hashes don't make a heading:
+
+####### over-enthusiastic heading
+
+So the detected heading should be right up at the start of this file.
+
+ChangeMe

userdiff.c

@@ -79,6 +79,9 @@ PATTERNS("java",
 	 "|[-+0-9.e]+[fFlL]?|0[xXbB]?[0-9a-fA-F]+[lL]?"
 	 "|[-+*/<>%&^|=!]="
 	 "|--|\\+\\+|<<=?|>>>?=?|&&|\\|\\|"),
+PATTERNS("markdown",
+	 "^ {0,3}#{1,6}[ \t].*",
+	 "[^<>= \t]+"),
 PATTERNS("matlab",
 	 /*
 	  * Octave pattern is mostly the same as matlab, except that '%%%' and