Enable SGR mouse support on mintty

[k-takata]

This was originally discussed at mintty/mintty#827.

Currently 'ttymouse' is set to "xterm" even mintty supports SGR mouse mode.
I think it is useful if Vim detects mintty and set 'ttymouse' to "sgr" automatically.

mintty 2.9.5 returns ESC[>77;20905;0c to v:termresponse. (77 is ASCII 'M'.)
This PR detects mintty by checking 77;.

[codecov-io]

Codecov Report

Merging #3667 into master will decrease coverage by <.01%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master    #3667      +/-   ##
==========================================
- Coverage    77.4%    77.4%   -0.01%     
==========================================
  Files          99       99              
  Lines      139423   139427       +4     
==========================================
- Hits       107924   107923       -1     
- Misses      31499    31504       +5

Impacted Files	Coverage Δ
src/term.c	60.34% <40%> (-0.08%)	⬇️
src/gui_gtk_x11.c	48.5% <0%> (-0.59%)	⬇️
src/if_xcmdsrv.c	83.63% <0%> (-0.54%)	⬇️
src/ex_docmd.c	79.75% <0%> (+0.01%)	⬆️
src/ex_cmds.c	80% <0%> (+0.02%)	⬆️
src/undo.c	83.18% <0%> (+0.06%)	⬆️
src/gui.c	58.06% <0%> (+0.51%)	⬆️

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