templater: add "diff" template function

"diff" allows to embed changes in the target revision into template
output, even if the command itself doesn't take "--patch" option

Combination of "[committemplate]" configuration and "diff" template
function can achieve the feature like issue231 ("option to have diff
displayed in commit editor buffer")

    http://bz.selenic.com/show_bug.cgi?id=231

For example, templating below can be used to add each "diff" output
lines "HG: " prefix::

      {splitlines(diff) % 'HG: {line}\n'}

This patch implements "diff" not as "a template keyword" but as "a
template function" to take include/exclude patterns at runtime.

It allows to specify target files of command (by -I/-X command line
options) and "diff" separately.

mercurial/help/templates.txt

@@ -43,6 +43,8 b' In addition to filters, there are some b'
 
 - date(date[, fmt])
 
+- diff([includepattern [, excludepattern]])
+
 - fill(text[, width])
 
 - get(dict, key)

mercurial/templater.py

@@ -225,6 +225,23 b' def date(context, mapping, args):'
         return util.datestr(date, fmt)
     return util.datestr(date)
 
+def diff(context, mapping, args):
+    if len(args) > 2:
+        # i18n: "diff" is a keyword
+        raise error.ParseError(_("diff expects one, two or no arguments"))
+
+    def getpatterns(i):
+        if i < len(args):
+            s = args[i][1].strip()
+            if s:
+                return [s]
+        return []
+
+    ctx = mapping['ctx']
+    chunks = ctx.diff(match=ctx.match([], getpatterns(0), getpatterns(1)))
+
+    return ''.join(chunks)
+
 def fill(context, mapping, args):
     if not (1 <= len(args) <= 4):
         raise error.ParseError(_("fill expects one to four arguments"))
@@ -516,6 +533,7 b' methods = {'
 
 funcs = {
     "date": date,
+    "diff": diff,
     "fill": fill,
     "get": get,
     "if": if_,

tests/test-command-template.t

@@ -1794,6 +1794,61 b' Thrown an error if a template function d'
   hg: parse error: unknown function 'foo'
   [255]
 
+Test diff function:
+
+  $ hg diff -c 8
+  diff -r 29114dbae42b -r 95c24699272e fourth
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/fourth	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +second
+  diff -r 29114dbae42b -r 95c24699272e second
+  --- a/second	Mon Jan 12 13:46:40 1970 +0000
+  +++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +0,0 @@
+  -second
+  diff -r 29114dbae42b -r 95c24699272e third
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/third	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +third
+
+  $ hg log -r 8 -T "{diff()}"
+  diff -r 29114dbae42b -r 95c24699272e fourth
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/fourth	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +second
+  diff -r 29114dbae42b -r 95c24699272e second
+  --- a/second	Mon Jan 12 13:46:40 1970 +0000
+  +++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +0,0 @@
+  -second
+  diff -r 29114dbae42b -r 95c24699272e third
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/third	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +third
+
+  $ hg log -r 8 -T "{diff('glob:f*')}"
+  diff -r 29114dbae42b -r 95c24699272e fourth
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/fourth	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +second
+
+  $ hg log -r 8 -T "{diff('', 'glob:f*')}"
+  diff -r 29114dbae42b -r 95c24699272e second
+  --- a/second	Mon Jan 12 13:46:40 1970 +0000
+  +++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +0,0 @@
+  -second
+  diff -r 29114dbae42b -r 95c24699272e third
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/third	Wed Jan 01 10:01:00 2020 +0000
+  @@ -0,0 +1,1 @@
+  +third
+
   $ cd ..