commit: show interesting ident information in summary

There are a few cases of user identity information that we consider
interesting:

  (1) When the author and committer identities do not match.

  (2) When the committer identity was picked automatically from the
      username, hostname and GECOS information.

In these cases, we already show the information in the commit
message template. However, users do not always see that template
because they might use "-m" or "-F". With this patch, we show these
interesting cases after the commit, along with the subject and
change summary. The new output looks like:

  $ git commit \
      -m "federalist papers" \
      --author='Publius <alexander@hamilton.com>'
  [master 3d226a7] federalist papers
   Author: Publius <alexander@hamilton.com>
   1 files changed, 1 insertions(+), 0 deletions(-)

for case (1), and:

  $ git config --global --unset user.name
  $ git config --global --unset user.email
  $ git commit -m foo
  [master 7c2a927] foo
   Committer: Jeff King <peff@c-71-185-130-222.hsd1.va.comcast.net>
  Your name and email address were configured automatically based
  on your username and hostname. Please check that they are accurate.
  You can suppress this message by setting them explicitly:

      git config --global user.name Your Name
      git config --global user.email you@example.com

  If the identity used for this commit is wrong, you can fix it with:

      git commit --amend --author='Your Name <you@example.com>'

   1 files changed, 1 insertions(+), 0 deletions(-)

for case (2).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -35,7 +35,20 @@ static const char * const builtin_status_usage[] = {
 	NULL
 };
 
+static const char implicit_ident_advice[] =
+"Your name and email address were configured automatically based\n"
+"on your username and hostname. Please check that they are accurate.\n"
+"You can suppress this message by setting them explicitly:\n"
+"\n"
+"    git config --global user.name Your Name\n"
+"    git config --global user.email you@example.com\n"
+"\n"
+"If the identity used for this commit is wrong, you can fix it with:\n"
+"\n"
+"    git commit --amend --author='Your Name <you@example.com>'\n";
+
 static unsigned char head_sha1[20], merge_head_sha1[20];
+
 static char *use_message_buffer;
 static const char commit_editmsg[] = "COMMIT_EDITMSG";
 static struct lock_file index_lock; /* real index */
@@ -957,9 +970,12 @@ static void print_summary(const char *prefix, const unsigned char *sha1)
 {
 	struct rev_info rev;
 	struct commit *commit;
-	static const char *format = "format:%h] %s";
+	struct strbuf format = STRBUF_INIT;
 	unsigned char junk_sha1[20];
 	const char *head = resolve_ref("HEAD", junk_sha1, 0, NULL);
+	struct pretty_print_context pctx = {0};
+	struct strbuf author_ident = STRBUF_INIT;
+	struct strbuf committer_ident = STRBUF_INIT;
 
 	commit = lookup_commit(sha1);
 	if (!commit)
@@ -967,6 +983,23 @@ static void print_summary(const char *prefix, const unsigned char *sha1)
 	if (!commit || parse_commit(commit))
 		die("could not parse newly created commit");
 
+	strbuf_addstr(&format, "format:%h] %s");
+
+	format_commit_message(commit, "%an <%ae>", &author_ident, &pctx);
+	format_commit_message(commit, "%cn <%ce>", &committer_ident, &pctx);
+	if (strbuf_cmp(&author_ident, &committer_ident)) {
+		strbuf_addstr(&format, "\n Author: ");
+		strbuf_addbuf_percentquote(&format, &author_ident);
+	}
+	if (!user_ident_explicitly_given) {
+		strbuf_addstr(&format, "\n Committer: ");
+		strbuf_addbuf_percentquote(&format, &committer_ident);
+		strbuf_addch(&format, '\n');
+		strbuf_addstr(&format, implicit_ident_advice);
+	}
+	strbuf_release(&author_ident);
+	strbuf_release(&committer_ident);
+
 	init_revisions(&rev, prefix);
 	setup_revisions(0, NULL, &rev, NULL);
 
@@ -977,7 +1010,8 @@ static void print_summary(const char *prefix, const unsigned char *sha1)
 
 	rev.verbose_header = 1;
 	rev.show_root_diff = 1;
-	get_commit_format(format, &rev);
+	get_commit_format(format.buf, &rev);
+	strbuf_release(&format);
 	rev.always_show_header = 0;
 	rev.diffopt.detect_rename = 1;
 	rev.diffopt.rename_limit = 100;
@@ -996,7 +1030,7 @@ static void print_summary(const char *prefix, const unsigned char *sha1)
 		struct pretty_print_context ctx = {0};
 		struct strbuf buf = STRBUF_INIT;
 		ctx.date_mode = DATE_NORMAL;
-		format_commit_message(commit, format + 7, &buf, &ctx);
+		format_commit_message(commit, format.buf + 7, &buf, &ctx);
 		printf("%s\n", buf.buf);
 		strbuf_release(&buf);
 	}

t/t7501-commit.sh

@@ -117,7 +117,11 @@ test_expect_success \
 test_expect_success \
 	"overriding author from command line" \
 	"echo 'gak' >file && \
-	 git commit -m 'author' --author 'Rubber Duck <rduck@convoy.org>' -a"
+	 git commit -m 'author' --author 'Rubber Duck <rduck@convoy.org>' -a >output 2>&1"
+
+test_expect_success \
+	"commit --author output mentions author" \
+	"grep Rubber.Duck output"
 
 test_expect_success PERL \
 	"interactive add" \