git-gui:handle the encoding of Git's output correctly

[dscho]

If we use 'eval exec $opt $cmdp $args' to execute git command,
tcl engine will convert the output of the git comand with the rule
system default code page to unicode.

But cp936 -> unicode conversion implicitly done by exec is not reversible.
So we have to use git_read instead.

Bug report and an original reproducer by Cloud Chou:
msysgit#302

Karsten Blees writes this code patch.
Cloud Chou find the reason of the bug.

Thanks-to: dscho
Thanks-to: patthoyts
Signed-off-by: Karsten Blees blees@dcon.de
Original-test-by: Cloud Chou 515312382@qq.com
Signed-off-by: Cloud Chou 515312382@qq.com
Signed-off-by: Johannes Schindelin johannes.schindelin@gmx.de

[dscho]

I just analyzed what commits need porting from msysGit's Git fork, and it seems only this one is necessary. The commits on maint-1.9 are backports with the exception of the home fix which is no longer necessary because we dropped the get_home_directory() patch in favor of setting the HOME environment variable in the Git wrapper (or in Git Bash).

Therefore, we are two Pull Requests (this one and #34) and a merging rebase to 2.3.2 short of a new tag...

[dscho]

Okay, I realize that it was overly bureaucratic to make this a Pull Request. We already had this as a Pull Request for msysGit's master branch and accepted it. Given the fact that we are all busy with things, I therefore just merge this myself.