[git] Git Extensions: Win32 error 487: Couldn't reserve space for cygwin's heap, Win32 error 0

If a reboot does not correct the problem (as suggested by Greg Hegwill's answer) then check your PATH for conflicting installation(s) of the msys-1.0.dll (and possibly other related DLLs).

In my particular situation MinGW's installation of msys has a copy of that DLL in its bin directory (<MinGW_Install_Path>\msys\1.0\bin), and it was listed in the PATH. Git's cmd directory was listed in the PATH, but its bin was not. (Git's version of msys-1.0.dll is in the bin directory. Apparently the default installation of MSys-Git does not add its bin to the PATH.)

A temporary fix was to add Git's bin directory to the PATH so that it appears before MinGW's paths. (A more permanent fix will likely involve sorting out the path conflicts between MinGW's msys and Git's and/or removing the duplicate msys installations.)