I finally resolved this. I can't say exactly what the source of the problem all along has been - in other words, why it hasn't worked with various git installations, etc. Though I expect the problem was the same; it doesn't make sense that I kept installing different broken Git packages.
At any rate, what solved it is this:
I had downloaded the
libssh2.dll binaries from the libssh2.org website, but the file that came with it was
libssh2-1.dll, which went undetected by git. It didn't occur to me rename it, but that might've worked.
However, what did work was downloading the Cgminer binaries, which - as I somehow discovered - happen to contain
I then copied
..\mysisgit\bin, and git has worked perfectly since. God knows why it wasn't there; especially since this is the version of mysisgit that is packaged with
cmder (thus I assume it is a recent mysisgit version).
This has driven me stupid. Notice the date of my original post.