Fix shell-command-dont-erase-buffer feature

* lisp/simple.el (shell-command-dont-erase-buffer):
The default, nil, is backward compatible, i.e. it erases the buffer
only if the output buffer is not the current one; the new value 'erase
always erases the output buffer.
Update docstring.

(shell-command-save-pos-or-erase):
Add optional arg output-to-current-buffer.
Rename it so that it's not internal.  All callers updated.

(shell-command-set-point-after-cmd): Rename it so that it's not internal.
All callers updated.
Adjust it to cover a side case.

(shell-command): Adjust logic to match the specification (Bug#39067).
Enable the feature when the output buffer is the current one.

(shell-command-on-region): Little tweak to follow
`shell-command-dont-erase-buffer' specification.

* test/lisp/simple-tests.el (with-shell-command-dont-erase-buffer):
Add helper macro.
(simple-tests-shell-command-39067)
(simple-tests-shell-command-dont-erase-buffer): Add tests.

* doc/emacs/misc.texi (Single Shell): Update manual.

* etc/NEWS (Single shell commands): Announce the change.

1 files changed, 8 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index a6092736cec..d06b2a2f205 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2038,6 +2038,14 @@ variable for remote shells.  It still defaults to "/bin/sh".
 ** Single shell commands
 
 +++
+*** 'shell-command-dont-erase-buffer' accepts the value 'erase' to
+force to erase the output buffer before execution of the command.
+
+*** The new functions shell-command-save-pos-or-erase' and
+'shell-command-set-point-after-cmd' control how point is handled
+between two consecutive shell commands in the same buffer.
+
++++
 *** 'async-shell-command-width' defines the number of display columns
 available for output of asynchronous shell commands.