git-p4: add config git-p4.pathEncoding

Perforce keeps the encoding of a path as given by the originating OS.
Git expects paths encoded as UTF-8. Add a config to tell git-p4 what
encoding Perforce had used for the paths. This encoding is used to
transcode the paths to UTF-8. As an example, Perforce on Windows often
uses “cp1252” to encode path names.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -2213,6 +2213,17 @@ class P4Sync(Command, P4UserMap):
             text = regexp.sub(r'$\1$', text)
             contents = [ text ]
 
+        if gitConfig("git-p4.pathEncoding"):
+            relPath = relPath.decode(gitConfig("git-p4.pathEncoding")).encode('utf8', 'replace')
+        elif self.verbose:
+            try:
+                relPath.decode('ascii')
+            except:
+                print (
+                    "Path with Non-ASCII characters detected and no path encoding defined. "
+                    "Please check the encoding: %s" % relPath
+                )
+
         self.gitStream.write("M %s inline %s\n" % (git_mode, relPath))
 
         # total length...