format-patch: have progress option while generating patches

When generating patches for the rebase command, if the user does
not realize the branch they are rebasing onto is thousands of
commits different, there is no progress indication after initial
rewinding message.

The progress meter as presented in this patch assumes the thousands of
patches to have a fine granularity as well as assuming to require all
the same amount of work/time for each, such that a steady progress bar
is achieved.

We do not want to estimate the time for each patch based e.g.
on their size or number of touched files (or parents) as that is too
expensive for just a progress meter.

This patch allows a progress option to be passed to format-patch
so that the user can be informed the progress of generating the
patch.  This option is then used by the rebase command when
calling format-patch.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -23,6 +23,7 @@ SYNOPSIS
 		   [(--reroll-count|-v) <n>]
 		   [--to=<email>] [--cc=<email>]
 		   [--[no-]cover-letter] [--quiet] [--notes[=<ref>]]
+		   [--progress]
 		   [<common diff options>]
 		   [ <since> | <revision range> ]
 
@@ -283,6 +284,9 @@ you can use `--suffix=-patch` to get `0001-description-of-my-change-patch`.
 	range are always formatted as creation patches, independently
 	of this flag.
 
+--progress::
+	Show progress reports on stderr as patches are generated.
+
 CONFIGURATION
 -------------
 You can specify extra mail header lines to be added to each message,

builtin/log.c

@@ -27,6 +27,7 @@
 #include "version.h"
 #include "mailmap.h"
 #include "gpg-interface.h"
+#include "progress.h"
 
 /* Set a default date-time format for git log ("log.date" config variable) */
 static const char *default_date_mode = NULL;
@@ -1419,6 +1420,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	char *branch_name = NULL;
 	char *base_commit = NULL;
 	struct base_tree_info bases;
+	int show_progress = 0;
+	struct progress *progress = NULL;
 
 	const struct option builtin_format_patch_options[] = {
 		{ OPTION_CALLBACK, 'n', "numbered", &numbered, NULL,
@@ -1490,6 +1493,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		OPT_FILENAME(0, "signature-file", &signature_file,
 				N_("add a signature from a file")),
 		OPT__QUIET(&quiet, N_("don't print the patch filenames")),
+		OPT_BOOL(0, "progress", &show_progress,
+			 N_("show progress while generating patches")),
 		OPT_END()
 	};
 
@@ -1749,8 +1754,12 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		start_number--;
 	}
 	rev.add_signoff = do_signoff;
+
+	if (show_progress)
+		progress = start_progress_delay(_("Generating patches"), total, 0, 2);
 	while (0 <= --nr) {
 		int shown;
+		display_progress(progress, total - nr);
 		commit = list[nr];
 		rev.nr = total - nr + (start_number - 1);
 		/* Make the second and subsequent mails replies to the first */
@@ -1815,6 +1824,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		if (!use_stdout)
 			fclose(rev.diffopt.file);
 	}
+	stop_progress(&progress);
 	free(list);
 	free(branch_name);
 	string_list_clear(&extra_to, 0);