mingw: special-case arguments to `sh`

mingw: special-case arguments to `sh`

The MSYS2 runtime does its best to emulate the command-line wildcard
expansion and de-quoting which would be performed by the calling Unix
shell on Unix systems.

Those Unix shell quoting rules differ from the quoting rules applying to
Windows' cmd and Powershell, making it a little awkward to quote
command-line parameters properly when spawning other processes.

In particular, git.exe passes arguments to subprocesses that are *not*
intended to be interpreted as wildcards, and if they contain
backslashes, those are not to be interpreted as escape characters, e.g.
when passing Windows paths.

Note: this is only a problem when calling MSYS2 executables, not when
calling MINGW executables such as git.exe. However, we do call MSYS2
executables frequently, most notably when setting the use_shell flag in
the child_process structure.

There is no elegant way to determine whether the .exe file to be
executed is an MSYS2 program or a MINGW one. But since the use case of
passing a command line through the shell is so prevalent, we need to
work around this issue at least when executing sh.exe.

Let's introduce an ugly, hard-coded test whether argv[0] is "sh", and
whether it refers to the MSYS2 Bash, to determine whether we need to
quote the arguments differently than usual.

That still does not fix the issue completely, but at least it is
something.

Incidentally, this also fixes the problem where `git clone \\server\repo`
failed due to incorrect handling of the backslashes when handing the path
to the git-upload-pack process.

Further, we need to take care to quote not only whitespace and
backslashes, but also curly brackets. As aliases frequently go through
the MSYS2 Bash, and as aliases frequently get parameters such as
HEAD@{yesterday}, this is really important. As an early version of this
patch broke this, let's make sure that this does not regress by adding a
test case for that.

Helped-by: Kim Gybels <kgybels@infogroep.be>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>