refactor(system-lib): extract cj/process-output-or-error and cj/git-output-or-error from coverage-core

Phase 2.3 of utility-consolidation. `cj/--coverage-git-string' was a generic argv-based runner ("run program, return stdout, raise user-error on non-zero with status+output in the message") trapped inside coverage-core.  Lift the generic shape into `cj/process-output-or-error' and add `cj/git-output-or-error' as a one-line wrapper that supplies "git" as the program.  Both live in system-lib.el.

Future callers I have in mind: reconcile-open-repos shell-style git calls (the high-priority data-safety task), vc-config clipboard cloning, mail integrations that touch git for commit signatures.

Six Normal/Boundary/Error tests cover success/no-args/non-zero-exit for the generic runner, the user-error message content (program name, exit status, trimmed output), and the git wrapper's program argument routing.

Migrate coverage-core's `cj/--coverage-git-merge-base' and `cj/--coverage-git-diff' to call the new git wrapper.  Drop the local `cj/--coverage-git-string' definition.  Add `(require \='system-lib)' to coverage-core.el per the Phase 2 exit criterion.

1 files changed, 26 insertions, 0 deletions

diff --git a/modules/system-lib.el b/modules/system-lib.el
index dc1f8316..3ccec06b 100644
--- a/modules/system-lib.el
+++ b/modules/system-lib.el
@@ -54,6 +54,32 @@ interpolate."
       argument
     (shell-quote-argument argument)))
 
+(defun cj/process-output-or-error (program &rest args)
+  "Run PROGRAM with ARGS via `process-file' and return stdout, or signal error.
+
+On zero exit, returns the program's stdout as a string (including any
+trailing newline -- callers that need a trimmed value should call
+`string-trim' themselves).  On non-zero exit, signals `user-error' with
+a message naming the program, the exit status, and the (trimmed) output
+so a user inspecting *Messages* can see what went wrong."
+  (with-temp-buffer
+    (let ((status (apply #'process-file program nil (current-buffer) nil args))
+          (output (buffer-string)))
+      (unless (zerop status)
+        (user-error "%s %s failed with status %s: %s"
+                    program
+                    (string-join args " ")
+                    status
+                    (string-trim output)))
+      output)))
+
+(defun cj/git-output-or-error (&rest args)
+  "Run git with ARGS and return stdout, or signal `user-error' on failure.
+
+Thin wrapper around `cj/process-output-or-error' with `git' as the
+program."
+  (apply #'cj/process-output-or-error "git" args))
+
 (defun cj/log-silently (format-string &rest args)
   "Append formatted message (FORMAT-STRING with ARGS) to *Messages* buffer.
 This does so without echoing in the minibuffer."