When using `git_process_new` on win32, resolve the path to the
application in the same way that we do on POSIX.
Search `PATH` for command to execute (unless the given executable is
fully qualified). In addition, better match Windows executable lookup
behavior itself (allowing the command to be `foo`, and looking for a
matching `foo.exe` or `foo.cmd`.)
In keeping with the libgit2 pattern, we _dispose_ a structure that is
not heap allocated, and _free_ a structure's contents _and the structure
itself_.
Our process tests were previously testing that false is
`/bin/false` everywhere except macOS, where it exists as
`/usr/bin/false`. . Extend this to all BSDs.
Provide both cmdline-style handling (passing it to the shell on POSIX,
or directly to CreateProcess on win32) and execv style (passing it
directly to execv on POSIX, and mangling it into a single command-line
on win32).
We can't reliably detect SIGPIPE on close because of platform
differences. Track `pid` and send `SIGTERM` to a function and ensure
that we can detect it.
