Start the merging-rebase to junio/maint

In preparation for v2.9.3, let's perform an intermediate rebase. This
rebase not only updated Git for Windows' `master` branch to upstream
Git's current `maint` branch, but also rearranges a couple of commits.

Most notably, a couple of patches that were previously not in topic
branches now are, and the recently merged Pull Requests are now rooted
in the upstream revision, not some semi-random Windows-specific patch.

In addition, these two topic branches (which both require the "Win32:
simplify loading of DLL functions" patch) were moved so that the code
actually compiles not only at the very end, but also when checking out
these revisions individually:

mingw/default-ident
	mingw: use domain information for default email
	getpwuid(mingw): provide a better default for the user name
	getpwuid(mingw): initialize the structure only once

mingw/getcwd
	mingw: fix getcwd when the parent directory cannot be queried
	mingw: ensure `getcwd()` reports the correct case

Finally, the patch "credential-store: avoid assertion" was replaced by a
merge of Jeff King's "common-main" branch that was already merged into
upstream Git's `master` branch.

This commit starts the rebase of b71d344 to 00f27fe

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho