templater: add separate() template function

A pretty common pattern in templates is adding conditional separators
like so:

  {node}{if(bookmarks, " {bookmarks}")}{if(tags, " {tags}")}

With this patch, the above can be simplified to:

  {separate(" ", node, bookmarks, tags)}

The function is similar to the already existing join(), but with a few
differences:

 * separate() skips empty arguments

 * join() expects a single list argument, while separate() expects
   each item as a separate argument

 * separate() takes the separator first in order to allow a variable
   number of arguments after it

mercurial/help/templates.txt

@@ -81,6 +81,10 Some sample command line templates:
 
    $ hg log -r 0 --template "files: {join(files, ', ')}\n"
 
+- Separate non-empty arguments by a " "::
+
+   $ hg log -r 0 --template "{separate(' ', node, bookmarks, tags}\n"
+
 - Modify each line of a commit description::
 
    $ hg log --template "{splitlines(desc) % '**** {line}\n'}"

mercurial/templater.py

@@ -724,6 +724,25 def rstdoc(context, mapping, args):
 
     return minirst.format(text, style=style, keep=['verbose'])
 
+@templatefunc('separate(sep, args)')
+def separate(context, mapping, args):
+    """Add a separator between non-empty arguments."""
+    if not args:
+        # i18n: "separate" is a keyword
+        raise error.ParseError(_("separate expects at least one argument"))
+
+    sep = evalstring(context, mapping, args[0])
+    first = True
+    for arg in args[1:]:
+        argstr = evalstring(context, mapping, arg)
+        if not argstr:
+            continue
+        if first:
+            first = False
+        else:
+            yield sep
+        yield argstr
+
 @templatefunc('shortest(node, minlength=4)')
 def shortest(context, mapping, args):
     """Obtain the shortest representation of

tests/test-command-template.t

@@ -3320,6 +3320,15 Test width argument passed to pad functi
   hg: parse error: pad() expects an integer width
   [255]
 
+Test separate function
+
+  $ hg log -r 0 -T '{separate("-", "", "a", "b", "", "", "c", "")}\n'
+  a-b-c
+  $ hg log -r 0 -T '{separate(" ", "{rev}:{node|short}", author|user, branch)}\n'
+  0:f7769ec2ab97 test default
+  $ hg log -r 0 --color=always -T '{separate(" ", "a", label(red, "b"), "c", label(red, ""), "d")}\n'
+  a \x1b[0;31mb\x1b[0m c d (esc)
+
 Test ifcontains function
 
   $ hg log --template '{rev} {ifcontains(rev, "2 two 0", "is in the string", "is not")}\n'