a couple of read_key_without_echo() fixes

I have added a new patch to the beginning of the series that fixes a case where we did not call restore_term() when leaving read_key_without_echo(). I have also reworded the commit message to patch 2 as SIGINT is actually ignored while the editor is running (we should probably change that code to use wait_after_clean instead).

Cover letter for V1:

Fix a couple of bugs I noticed when using the builtin "add -p" with interactive.singlekey=true. The first patch is a general fix for the terminal save/restore functionality which forgot to call sigchain_pop() when it restored the terminal. The last two fix reading single keys in non-canonical mode by making sure we wait for an initial key press and only read one byte at a time from the underlying file descriptor.

Note that these are untested on windows beyond our CI compiling them

Phillip Wood (4):
terminal: always reset terminal when reading without echo
terminal: pop signal handler when terminal is restored
terminal: set VMIN and VTIME in non-canonical mode
add -p: disable stdin buffering when interactive.singlekey is set

 add-interactive.c |  2 ++
 compat/terminal.c | 29 +++++++++++++++++++++++------
 compat/terminal.h |  8 ++++++++
 3 files changed, 33 insertions(+), 6 deletions(-)

base-commit: b80121027d1247a0754b3cc46897fee75c050b44
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1146%2Fphillipwood%2Fwip%2Fadd-p-vmin-v2-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1146/phillipwood/wip/add-p-vmin-v2-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1146

Range-diff vs v1:

 -:  ----------- > 1:  45609d61afc terminal: always reset terminal when reading without echo
 1:  9a3c2cea0f9 ! 2:  0020953f1cf terminal: pop signal handler when terminal is restored

@@ Commit message
external caller was removed by e3f7e01b50 ("Revert "editor: save and reset terminal after calling EDITOR"", 2021-11-22). Any future callers of save_term() should benefit from having the signal handler set up
- for them. For example if we reinstate the code to protect against an
- editor failing to restore the terminal attributes we would want that
- code to restore the terminal attributes on SIGINT. (As an aside
- run_command() installs a signal handler that waits for the child
- before re-raising the signal so we would be guaranteed to restore the
- settings after the child has exited.)
+ for them.

 2:  02009172e08 = 3:  7ae9b236554 terminal: set VMIN and VTIME in non-canonical mode
 3:  6d8423b6e1e = 4:  39b061a471b add -p: disable stdin buffering when interactive.singlekey is set

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