import-checker: add xargs like mode

Before this patch, "import-check.py" is invoked via "xargs" in
"test-module-imports.t", but it doesn't ensure that
"import-checker.py" is certainly invoked with all mercurial specific
files at once.

"xargs" may invoke specified command multiple times with part of
arguments given from stdin: according to "xargs(1)" man page, this
dividing arguments is system-dependent.

This patch adds "xargs" like mode to "import-checker.py".

This can ensure that "import-checker.py" is certainly invoked with all
mercurial specific files at once in "test-module-imports.t". This is
assumed by subsequent patches.

contrib/import-checker.py

@@ -215,9 +215,12 b' def _cycle_sortkey(c):'
     return len(c), c
 
 def main(argv):
-    if len(argv) < 2:
-        print 'Usage: %s file [file] [file] ...'
+    if len(argv) < 2 or (argv[1] == '-' and len(argv) > 2):
+        print 'Usage: %s {-|file [file] [file] ...}'
         return 1
+    if argv[1] == '-':
+        argv = argv[:1]
+        argv.extend(l.rstrip() for l in sys.stdin.readlines())
     used_imports = {}
     any_errors = False
     for source_path in argv[1:]:

tests/test-module-imports.t

@@ -20,7 +20,7 b' here that we should still endeavor to fi'
 hidden by deduplication algorithm in the cycle detector, so fixing
 these may expose other cycles.
 
-  $ hg locate 'mercurial/**.py' | sed 's-\\-/-g' | xargs python "$import_checker"
+  $ hg locate 'mercurial/**.py' | sed 's-\\-/-g' | python "$import_checker" -
   mercurial/dispatch.py mixed imports
      stdlib:    commands
      relative:  error, extensions, fancyopts, hg, hook, util