i18n: check equality of initial indentation between msgid and msgstr

Document generation by runrst in "doc" directory may succeed silently,
even though initial indentation is different between msgid and msgstr:
for example, it may be unexpected or missing indentation.

This patch adds the checker to check equality of initial indentation
between msgid and msgstr.

This checker is categorized as "warning" level, because problem
detected by this is not so serious for usual Mercurial usage.

i18n/check-translation.py

@@ -88,6 +88,24 b' def taildoublecolons(pe):'
     if pe.msgid.endswith('::') != pe.msgstr.endswith('::'):
         yield "tail '::'-ness differs between msgid and msgstr"
 
+@warningchecker()
+def indentation(pe):
+    """Check equality of initial indentation between msgid and msgstr
+
+    This may report unexpected warning, because this doesn't aware
+    the syntax of rst document and the context of msgstr.
+
+    >>> pe = polib.POEntry(
+    ...     msgid ='    indented text',
+    ...     msgstr='  narrowed indentation')
+    >>> for e in indentation(pe): print e
+    initial indentation width differs betweeen msgid and msgstr
+    """
+    idindent = len(pe.msgid) - len(pe.msgid.lstrip())
+    strindent = len(pe.msgstr) - len(pe.msgstr.lstrip())
+    if idindent != strindent:
+        yield "initial indentation width differs betweeen msgid and msgstr"
+
 ####################
 
 def check(pofile, fatal=True, warning=False):