Fixes #1731

Major changes:

• Remove all uses of is_win and is_posix in git/ and test/, replacing them with os.name == "nt" and os.name == "posix", respectively.
• Fix is_darwin by changing it to use sys.platform instead of os.name which is never "darwin".
• Add a comment in the git.compat module above the definitions of is_win, is_posix, and is_darwin stating that they are deprecated and recommending that os.name (or, where applicable, sys.platform) be used directly instead for clarity (and sometimes accuracy).

There were no uses of is_darwin to be replaced. Although it had been used at one time, the last reference to it appears to have been removed in 4545762 (#1295).

This doesn't emit a DeprecationWarning when those attributes are accessed from the git.compat module. (That might be valuable thing to do in the future, if the git.compat module is to remain non-deprecated overall. See discussion in #1731.)

Two related less consequential changes are also included:

• Improve ordering and grouping of imports, in modules where they were already being changed as a result of no longer needing to import is_<platform> (usually is_win) from git.compat.
• Make minor revisions to a few comments, for readability. (This is in addition to somewhat more substantial revisions of comments where rewording was related to replacing uses of is_<platform>.)