convert: for git's getchanges, use explicit index for iteration

Upcoming patches will add support for copies and renames, for which we'll need
to access multiple lines of the difftree output at once.

hgext/convert/git.py

@@ -190,7 +190,12 b' class convert_git(converter_source):'
         entry = None
         subexists = False
         subdeleted = False
-        for l in fh.read().split('\x00'):
+        difftree = fh.read().split('\x00')
+        lcount = len(difftree)
+        i = 0
+        while i < lcount:
+            l = difftree[i]
+            i += 1
             if not entry:
                 if not l.startswith(':'):
                     continue