pretty: implement 'reference' format

The standard format for referencing other commits within some projects
(such as git.git) is the reference format. This is described in
Documentation/SubmittingPatches as

	If you want to reference a previous commit in the history of a stable
	branch, use the format "abbreviated hash (subject, date)", like this:

	....
		Commit f86a374 (pack-bitmap.c: fix a memleak, 2015-03-30)
		noticed that ...
	....

Since this format is so commonly used, standardize it as a pretty
format.

The tests that are implemented essentially show that the format-string
does not change in response to various log options. This is useful
because, for future developers, it shows that we've considered the
limitations of the "canned format-string" approach and we are fine with
them.

Based-on-a-patch-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -63,6 +63,17 @@ This is designed to be as compact as possible.
 
 	       <full commit message>
 
+* 'reference'
+
+	  <abbrev hash> (<title line>, <short author date>)
++
+This format is used to refer to another commit in a commit message and
+is the same as `--pretty='format:%C(auto)%h (%s, %ad)'`.  By default,
+the date is formatted with `--date=short` unless another `--date` option
+is explicitly specified.  As with any `format:` with format
+placeholders, its output is not affected by other options like
+`--decorate` and `--walk-reflogs`.
+
 * 'email'
 
 	  From <hash> <date>

Documentation/pretty-options.txt

@@ -3,7 +3,7 @@
 
 	Pretty-print the contents of the commit logs in a given format,
 	where '<format>' can be one of 'oneline', 'short', 'medium',
-	'full', 'fuller', 'email', 'raw', 'format:<string>'
+	'full', 'fuller', 'reference', 'email', 'raw', 'format:<string>'
 	and 'tformat:<string>'.  When '<format>' is none of the above,
 	and has '%placeholder' in it, it acts as if
 	'--pretty=tformat:<format>' were given.

Documentation/rev-list-options.txt

@@ -269,7 +269,7 @@ list.
 	exclude (that is, '{caret}commit', 'commit1..commit2',
 	and 'commit1\...commit2' notations cannot be used).
 +
-With `--pretty` format other than `oneline` (for obvious reasons),
+With `--pretty` format other than `oneline` and `reference` (for obvious reasons),
 this causes the output to have two extra lines of information
 taken from the reflog.  The reflog designator in the output may be shown
 as `ref@{Nth}` (where `Nth` is the reverse-chronological index in the
@@ -293,6 +293,8 @@ Under `--pretty=oneline`, the commit message is
 prefixed with this information on the same line.
 This option cannot be combined with `--reverse`.
 See also linkgit:git-reflog[1].
++
+Under `--pretty=reference`, this information will not be shown at all.
 
 --merge::
 	After a failed merge, show refs that touch files having a

contrib/completion/git-completion.bash

@@ -1737,7 +1737,7 @@ __git_log_shortlog_options="
 	--all-match --invert-grep
 "
 
-__git_log_pretty_formats="oneline short medium full fuller email raw format: tformat: mboxrd"
+__git_log_pretty_formats="oneline short medium full fuller reference email raw format: tformat: mboxrd"
 __git_log_date_formats="relative iso8601 iso8601-strict rfc2822 short local default raw unix format:"
 
 _git_log ()

pretty.c

@@ -98,7 +98,9 @@ static void setup_commit_formats(void)
 		{ "mboxrd",	CMIT_FMT_MBOXRD,	0,	0 },
 		{ "fuller",	CMIT_FMT_FULLER,	0,	8 },
 		{ "full",	CMIT_FMT_FULL,		0,	8 },
-		{ "oneline",	CMIT_FMT_ONELINE,	1,	0 }
+		{ "oneline",	CMIT_FMT_ONELINE,	1,	0 },
+		{ "reference",	CMIT_FMT_USERFORMAT,	1,	0,
+			0, DATE_SHORT, "%C(auto)%h (%s, %ad)" },
 		/*
 		 * Please update $__git_log_pretty_formats in
 		 * git-completion.bash when you add new formats.

t/t4205-log-pretty-formats.sh

@@ -824,4 +824,47 @@ test_expect_success '%S in git log --format works with other placeholders (part
 	test_cmp expect actual
 '
 
+test_expect_success 'log --pretty=reference' '
+	git log --pretty="tformat:%h (%s, %as)" >expect &&
+	git log --pretty=reference >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --pretty=reference with log.date is overridden by short date' '
+	git log --pretty="tformat:%h (%s, %as)" >expect &&
+	test_config log.date rfc &&
+	git log --pretty=reference >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --pretty=reference with explicit date overrides short date' '
+	git log --date=rfc --pretty="tformat:%h (%s, %ad)" >expect &&
+	git log --date=rfc --pretty=reference >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --pretty=reference is never unabbreviated' '
+	git log --pretty="tformat:%h (%s, %as)" >expect &&
+	git log --no-abbrev-commit --pretty=reference >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --pretty=reference is never decorated' '
+	git log --pretty="tformat:%h (%s, %as)" >expect &&
+	git log --decorate=short --pretty=reference >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --pretty=reference does not output reflog info' '
+	git log --walk-reflogs --pretty="tformat:%h (%s, %as)" >expect &&
+	git log --walk-reflogs --pretty=reference >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --pretty=reference is colored appropriately' '
+	git log --color=always --pretty="tformat:%C(auto)%h (%s, %as)" >expect &&
+	git log --color=always --pretty=reference >actual &&
+	test_cmp expect actual
+'
+
 test_done