Start the merging-rebase to v2.7.0

Unlike the most recent merging-bases, the purpose of *this* merging
rebase is to clean up the branch structure a bit, including an early
merge-back of the Git for Windows patches we contributed upstream
in the past weeks. This should make it much easier to prepare the
next batches of patches to beam upstream.

In particular, we drop these commits:

- criss cross rename failure workaround

	It was merged in a different form upstream and we carried
	a bogus patch for a long time

- Special-case the MSYS2_TZ environment variable

	In the meantime, Git for Windows' fork of the MSYS2 runtime
	learned to pass on valid TZ values. So this work-around is
	no longer needed.

- am: Use cat instead of echo to avoid DOS line-endings (fixes t4150)

	`am` became a builtin last year, hence this patch is no longer
	necessary.

- Teach 'git pull' to handle --rebase=interactive
- Handle the branch.<name>.rebase value 'interactive'
- Teach 'git remote' that the config var branch.*.rebase can be 'interactive'

	The --rebase=interactive code we carried in Git for Windows for
	such a long time patched the shell version of `git pull`, but
	in the meantime upstream has a builtin version, so these changes
	were bogus. Even better: this developer ported the changes to
	the builtin `pull` and they were already accepted upstream.

- t9100: don't use symlinks with SVN on MINGW
- t0200: disable more tests with MSys2 that rely on locale.exe
- Skip t0204 for MinGW Git

	Turns out that these are actually no longer needed to pass the
	test suite.

- mingw: do not special-case .exe files anymore

	When submitting the batch of patches that fixes the test
	failures in Git for Windows' SDK, this patch was made obsolete
	by the patch that provided a putty.exe for testing that is
	*actually* a .exe (and no shell script).

- mingw: Prepare the TMP environment variable for shell scripts

	This patch was actually forward-ported to apply *after*
	Karsten's environment refactoring.

- mingw: Fix CVS-related tests
- t7800: configure $(pwd) for posix-paths on MINGW

	These patches were redone using the $PWD variable instead of
	re-overriding `pwd`.

- mingw (MSys1): enable support for perl regular expressions

	msysGit is dead. No need to carry a patch for it anymore.

This commit starts the rebase of abf3368 to fb59a3e

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho