t3901: test git-am encoding conversion

t3901: test git-am encoding conversion

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
supported the --utf8 and --no-utf8 options, and if set, would pass the
-u flag and the -k flag respectively.

git mailinfo -u will re-code the commit log message and authorship info
in the charset specified by i18n.commitencoding setting, while
git mailinfo -n will disable the re-coding.

Since d84029b (--utf8 is now default for 'git-am', 2007-01-08), --utf8
is set by default in git-am.

Add various encoding conversion tests to t3901 to test git-mailinfo's
encoding conversion. In addition, add a test for --no-utf8 to check that
no encoding conversion will occur if that option is set.

Helped-by: Johannes Sixt <redacted>
Reviewed-by: Stefan Beller <redacted>
Reviewed-by: Johannes Schindelin <redacted>
Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>