git: Always use OpenSSL on win32

By default, libssh2 uses Windows Cryptography Next Generation when targeting
win32. The wincng backend does not support ED25519, which is a widely-used
algorithm among git hosting services, and in some cases may be the only option
on remotes in certain configurations.

This change necessitates Windows users building jj to install openssl via vcpkg
for the target triple x64-windows-static-md when building for MSVC, or otherwise
have OpenSSL available at build time. This should also generally work when
building for MinGW, though I did not try it.

Fixes jj-vcs#3322

Cargo.toml

@@ -60,6 +60,7 @@ indoc = "2.0.4"
 insta = { version = "1.38.0", features = ["filters"] }
 itertools = "0.12.1"
 libc = { version = "0.2.153" }
+libssh2-sys = { version = "0.3.0", features = ["openssl-on-win32"] }
 maplit = "1.0.2"
 minus = { version = "5.6.1", features = ["dynamic_output", "search"] }
 num_cpus = "1.16.0"

lib/Cargo.toml

@@ -49,6 +49,7 @@ hex = { workspace = true }
 ignore = { workspace = true }
 itertools = { workspace = true }
 jj-lib-proc-macros = { workspace = true }
+libssh2-sys = { workspace = true }
 maplit = { workspace = true }
 once_cell = { workspace = true }
 pest = { workspace = true }