Crash on MSYS2 with GIT_WORK_TREE

[valtron <valtron2000@gmail.com>]

Git 1.12.0. When GIT_WORK_TREE contains a drive-letter and
forward-slashes, some git commands crash:

C:\repo>set GIT_WORK_TREE=C:/repo
C:\repo>git rev-parse HEAD
     1 [main] git 2332 cygwin_exception::open_stackdumpfile: Dumping
stack trace to git.exe.stackdump
C:\repo>set GIT_WORK_TREE=
C:\repo>git rev-parse HEAD
a394e40861e1012a96f9578a1f0cf0c5a49ede11

On the other hand, "C:\repo" and "/c/repo" don't have this issue.

Stacktrace from GDB (on git-rev-parse) is:

#0  0x000000018019634d in strcmp (s1=0x600062080 "/c/repo", s2=0x0)
   at /msys_scripts/msys2-runtime/src/msys2-runtime/newlib/libc/string/strcmp.c:83
#1  0x00000001005239f1 in ?? ()
#2  0x0000000100523f36 in ?? ()
#3  0x000000010046c6fa in ?? ()
#4  0x0000000100401b6d in ?? ()
#5  0x0000000100401e4b in ?? ()
#6  0x0000000100563593 in ?? ()
#7  0x0000000180047c37 in _cygwin_exit_return ()
   at /msys_scripts/msys2-runtime/src/msys2-runtime/winsup/cygwin/dcrt0.cc:1031
#8  0x0000000180045873 in _cygtls::call2 (this=0xffffce00,
func=0x180046bd0 <dll_crt0_1(void*)>, arg=0x0,
   buf=buf@entry=0xffffcdf0) at
/msys_scripts/msys2-runtime/src/msys2-runtime/winsup/cygwin/cygtls.cc:40
#9  0x0000000180045924 in _cygtls::call (func=<optimized out>,
arg=<optimized out>)
   at /msys_scripts/msys2-runtime/src/msys2-runtime/winsup/cygwin/cygtls.cc:27
#10 0x0000000000000000 in ?? ()
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

It seems "C:/repo" was changed to "/c/repo", but it crashes because it
gets compared to a nullptr.