Fix git branch name never shown as dirty

[jankatins]

The problem was that io.popen() returns a file handle and not the return code of the
called program. The problem was introduced by 567889e.

The new code was inspired by
http://stackoverflow.com/a/14031974/1380673

Closes: #971
Closes: #928

[jankatins]

A different way would be to use the hg way (read lines and see if there are any) with git status -s -uno.

[sveggiani]

Hi @JanSchulz.
I've done some test and this works. However, if I run the git diff --quiet --ignore-submodules HEAD 2 commnad with git version 2.7.4.windows.1 the output is always:

"fatal: ambiguous argument '2': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'"

I'm a little curious about why it works. Do you know why?

Thanks!

[jankatins]

@sveggiani the 2 is not part of the git command, but part of the redirection: in this case it means that the file handle 2 (stderr, which gets the error output) should be redirected to the special file NUL (aka "into nowhere"). In effect this means that error output should be hidden.

See here fore more info: https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/redirection.mspx?mfr=true

[sveggiani]

@JanSchulz ohh, I see. Thank you very much ;)