diff: turn on rename detection progress reporting

Since all of the progress happens before we generate any
output, this looks OK, even when output goes to a pager.
We do the usual --progress/--no-progress options and check
isatty(2) to enable the feature.

The argument parsing is a little ad-hoc, but we currently
have no parse-options infrastructure here at all.  However,
it should be safe to parse like this, because the prior
call to setup_revisions will have removed any options that
take an argument, and our parsing removes --progress from
argv for later parsers. The one exception is diff_no_index,
which may get called early, and needs to learn to ignore
--progress.

Signed-off-by: Jeff King <peff@peff.net>

Author: peff

Documentation/git-diff.adoc

@@ -152,6 +152,13 @@ section "3-Way Merge" for detailed information.
 	"Unmerged".  Can be used only when comparing the working tree
 	with the index.
 
+`--no-progress`::
+`--progress`::
+	Disable or enable progress reporting during long computations;
+	the default is to enable progress reporting when stderr is a
+	terminal. Currently the only computation with progress support
+	is inexact rename detection.
+
 `<path>...`::
 	The _<path>_ parameters, when given, are used to limit
 	the diff to the named paths (you can give directory

builtin/diff.c

@@ -410,6 +410,8 @@ int cmd_diff(int argc,
 	int nongit = 0, no_index = 0;
 	int result;
 	struct symdiff sdiff;
+	int progress = -1;
+	int unknown_argc, parsed_argc;
 
 	/*
 	 * We could get N tree-ish in the rev.pending_objects list.
@@ -539,6 +541,32 @@ int cmd_diff(int argc,
 		diff_setup_done(&rev.diffopt);
 	}
 
+	parsed_argc = 0;
+	for (unknown_argc = i = 1; i < argc; i++) {
+		const char *arg = argv[i];
+		if (!strcmp(arg, "--") || arg[0] != '-') {
+			int j;
+			for (j = i; j < argc; j++)
+				argv[unknown_argc++] = argv[j];
+			break;
+		}
+		else if (!strcmp(argv[i], "--progress"))
+			progress = 1;
+		else if (!strcmp(argv[i], "--no-progress"))
+			progress = 0;
+		else {
+			argv[unknown_argc++] = argv[i];
+			continue;
+		}
+		parsed_argc++;
+	}
+	argc -= parsed_argc;
+
+	if (progress == -1)
+		progress = isatty(2);
+	if (progress)
+		rev.diffopt.show_rename_progress = 1;
+
 	rev.diffopt.flags.recursive = 1;
 	rev.diffopt.rotate_to_strict = 1;