See JWilliams' answer for where to report bugs to GitHub. [Edit: perhaps this should be an answer to your other question.]
For what it's worth, it's not a good idea to use anything other than UTF-8 for the author and committer name encoding—the encoding field of the header is too difficult to apply to the pre-body part of the header, since it comes at the end of the lines:
>>> import subprocess
>>> p = subprocess.Popen(['git', 'cat-file', '-p', 'HEAD'], stdout=subprocess.PIPE)
>>> o = p.stdout.read()
>>> hdr, body = o.split('\n\n', 1)
>>> hdr = hdr.splitlines()
The header lines are long, even after splitting:
>>> import pprint
>>> pprint.pprint(hdr)
['tree 79036d838fc5ce13e849949d02e6048c2d33c561',
'author \xc5\x99\x89\x83@\xc8\x96\x97\x97\x85\x99 <\x88\x96\x97\x97\x85\x99|\x96\x94\x95\x89\x86\x81\x99\x89\x96\xa4\xa2K\x96\x99\x87> 1528844508 -0700',
'committer \xc5\x99\x89\x83@\xc8\x96\x97\x97\x85\x99 <\x88\x96\x97\x97\x85\x99|\x96\x94\x95\x89\x86\x81\x99\x89\x96\xa4\xa2K\x96\x99\x87> 1528844508 -0700',
'encoding cp037']
but we can see that the encoding comes last. If the encoding were something that had byte-codes that resembled newlines (cp037 doesn't, fortunately) we would not be able to parse the header itself.
For the body, however, it's a good idea to use the header's encoding information. If we work in something that does have the encoding available, well:
>>> body.decode('cp037')
u'Well, this should be interesting.\x8e'
(Python 2.7 here of course).
Obviously neither GitHub nor my Git on this machine can do this for cp037, but on this particular host, that's not surprising:
$ iconv -f cp037
iconv: conversion from cp037 unsupported
On another machine that has the character set installed, iconv does work. I did not try this commit in Git there, but I did feed a header-line byte string through it; the result was:
>>> import subprocess
>>> p = subprocess.Popen(['iconv', '-f', 'cp037'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
>>> so, se = p.communicate(s)
>>> so
'Eric Hopper\xc2\x80\x14hopper@omnifarious.org\xc2\x9e'
As you can see, the angle brackets have been damaged in translation (because the parse here was overly simple—we'd have to carefully avoid translating them). Again, though, the hazards are obvious: what if the encoding produces >?