templater: introduce startswith function

This function returns a string only if it starts with a given string.
It is particularly useful when combined with splitlines and/or used with
conditionals that fail when empty strings are passed in to take action
based on the contents of a line.

mercurial/help/templates.txt

@@ -66,6 +66,8 b' In addition to filters, there are some b'
 
 - shortest(node)
 
+- startswith(string, text)
+
 - strip(text[, chars])
 
 - sub(pat, repl, expr)
@@ -124,3 +126,7 b' Some sample command line templates:'
 - Mark the working copy parent with '@'::
 
    $ hg log --template "{ifcontains(rev, revset('.'), '@')}\n"
+
+- Show only commit descriptions that start with "template"::
+
+   $ hg log --template "{startswith(\"template\", firstline(desc))}\n"

mercurial/templater.py

@@ -466,6 +466,17 b' def sub(context, mapping, args):'
     src = stringify(_evalifliteral(args[2], context, mapping))
     yield re.sub(pat, rpl, src)
 
+def startswith(context, mapping, args):
+    if len(args) != 2:
+        raise error.ParseError(_("startswith expects two arguments"))
+
+    patn = stringify(args[0][0](context, mapping, args[0][1]))
+    text = stringify(args[1][0](context, mapping, args[1][1]))
+    if text.startswith(patn):
+        return text
+    return ''
+
+
 methods = {
     "string": lambda e, c: (runstring, e[1]),
     "rawstring": lambda e, c: (runrawstring, e[1]),
@@ -490,6 +501,7 b' funcs = {'
     "revset": revset,
     "rstdoc": rstdoc,
     "shortest": shortest,
+    "startswith": startswith,
     "strip": strip,
     "sub": sub,
 }

tests/test-command-template.t

@@ -1884,3 +1884,30 b' Test splitlines'
   |  foo other 3
   o  foo line 1
      foo line 2
+
+Test startswith
+  $ hg log -Gv -R a --template "{startswith(desc)}"
+  hg: parse error: startswith expects two arguments
+  [255]
+
+  $ hg log -Gv -R a --template "{startswith('line', desc)}"
+  @
+  |
+  o
+  |
+  o
+  
+  o
+  |\
+  | o
+  | |
+  o |
+  |/
+  o
+  |
+  o
+  |
+  o
+  |
+  o  line 1
+     line 2