templater: add relpath() to convert repo path to relative path (issue5394)

File paths in template are repository-absolute paths. This function can be
used to convert them to filesystem paths relative to cwd. This also converts
'/' to '\\' on Windows.

mercurial/templater.py

@@ -713,6 +713,18 b' def localdate(context, mapping, args):'
         tzoffset = util.makedate()[1]
     return (date[0], tzoffset)
 
+@templatefunc('relpath(path)')
+def relpath(context, mapping, args):
+    """Convert a repository-absolute path into a filesystem path relative to
+    the current working directory."""
+    if len(args) != 1:
+        # i18n: "relpath" is a keyword
+        raise error.ParseError(_("relpath expects one argument"))
+
+    repo = mapping['ctx'].repo()
+    path = evalstring(context, mapping, args[0])
+    return repo.pathto(path)
+
 @templatefunc('revset(query[, formatargs...])')
 def revset(context, mapping, args):
     """Execute a revision set query. See

tests/test-command-template.t

@@ -3521,6 +3521,15 b' Test files function'
   0
   
 
+Test relpath function
+
+  $ hg log -r0 -T '{files % "{file|relpath}\n"}'
+  a
+  $ cd ..
+  $ hg log -R r -r0 -T '{files % "{file|relpath}\n"}'
+  r/a (glob)
+  $ cd r
+
 Test active bookmark templating
 
   $ hg book foo