Merge commit 'db71b31827' from emacs upstream (2018-04-14)

[harryfei]

• port b300052: Try and speed up byte<->char conversion with many markers.
• port ed962f2b: Fix bug#30846, along with misc cleanups found along the way
• port d06c2e7: Add a history variable for `read-variable'
• port 6f66a43d7: Centralize Bug#30931 fix

[harryfei]

Need to fix the errors in tests.

[shaleh]

Once this tests are working this looks reasonable.

[agraven]

Odd, it's complaining about not having rustfmt installed

[shaleh]

Yeah, rustfmt is no longer preview. I fixed this recently. This PR needs to rebase to get it.

[agraven]

There's a segfault happening:

Backtrace:
../../src/bootstrap-emacs[0x51fc32]
../../src/bootstrap-emacs[0x5082e4]
../../src/bootstrap-emacs[0x51e78e]
../../src/bootstrap-emacs[0x51ea83]
../../src/bootstrap-emacs[0x51eac0]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x10330)[0x2acc138a9330]
../../src/bootstrap-emacs(unchain_marker+0x1f)[0x61c1af]
../../src/bootstrap-emacs[0x52c28a]
../../src/bootstrap-emacs[0x52c701]
../../src/bootstrap-emacs(insert_1_both+0x241)[0x52c961]
../../src/bootstrap-emacs[0x622525]
../../src/bootstrap-emacs[0x5f69ad]
../../src/bootstrap-emacs[0x5f6885]
Collecting OKURI-NASI entries...48%
make[3]: *** [../../lisp/cedet/semantic/bovine/c-by.el] Segmentation fault (core dumped)
make[3]: Leaving directory `/home/travis/build/remacs/remacs/admin/grammars'
make[2]: *** [semantic] Error 2
make[2]: *** Waiting for unfinished jobs....

[harryfei]

Can't resolve the issue by myself. Welcome anyone to work on it.