Allow custom "comment char"

Some users do want to write a line that begin with a pound sign, #,
in their commit log message.  Many tracking system recognise
a token of #<bugid> form, for example.

The support we offer these use cases is not very friendly to the end
users.  They have a choice between

 - Don't do it.  Avoid such a line by rewrapping or indenting; and

 - Use --cleanup=whitespace but remove all the hint lines we add.

Give them a way to set a custom comment char, e.g.

    $ git -c core.commentchar="%" commit

so that they do not have to do either of the two workarounds.

[jc: although I started the topic, all the tests and documentation
updates, many of the call sites of the new strbuf_add_commented_*()
functions, and the change to git-submodule.sh scripted Porcelain are
from Ralf.]

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0030-stripspace.sh

@@ -397,4 +397,39 @@ test_expect_success 'strip comments, too' '
 	test -z "$(echo "# comment" | git stripspace -s)"
 '
 
+test_expect_success 'strip comments with changed comment char' '
+	test ! -z "$(echo "; comment" | git -c core.commentchar=";" stripspace)" &&
+	test -z "$(echo "; comment" | git -c core.commentchar=";" stripspace -s)"
+'
+
+test_expect_success '-c with single line' '
+	printf "# foo\n" >expect &&
+	printf "foo" | git stripspace -c >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '-c with single line followed by empty line' '
+	printf "# foo\n#\n" >expect &&
+	printf "foo\n\n" | git stripspace -c >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '-c with newline only' '
+	printf "#\n" >expect &&
+	printf "\n" | git stripspace -c >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '--comment-lines with single line' '
+	printf "# foo\n" >expect &&
+	printf "foo" | git stripspace -c >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '-c with changed comment char' '
+	printf "; foo\n" >expect &&
+	printf "foo" | git -c core.commentchar=";" stripspace -c >actual &&
+	test_cmp expect actual
+'
+
 test_done

t/t7502-commit.sh

@@ -447,4 +447,11 @@ use_template="-t template"
 
 try_commit_status_combo
 
+test_expect_success 'commit --status with custom comment character' '
+	test_when_finished "git config --unset core.commentchar" &&
+	git config core.commentchar ";" &&
+	try_commit --status &&
+	test_i18ngrep "^; Changes to be committed:" .git/COMMIT_EDITMSG
+'
+
 test_done

t/t7508-status.sh

@@ -1253,6 +1253,56 @@ test_expect_success ".git/config ignore=dirty doesn't suppress submodule summary
 	git config -f .gitmodules  --remove-section submodule.subname
 '
 
+cat > expect << EOF
+; On branch master
+; Changes to be committed:
+;   (use "git reset HEAD <file>..." to unstage)
+;
+;	modified:   sm
+;
+; Changes not staged for commit:
+;   (use "git add <file>..." to update what will be committed)
+;   (use "git checkout -- <file>..." to discard changes in working directory)
+;
+;	modified:   dir1/modified
+;	modified:   sm (new commits)
+;
+; Submodule changes to be committed:
+;
+; * sm $head...$new_head (1):
+;   > Add bar
+;
+; Submodules changed but not updated:
+;
+; * sm $new_head...$head2 (1):
+;   > 2nd commit
+;
+; Untracked files:
+;   (use "git add <file>..." to include in what will be committed)
+;
+;	.gitmodules
+;	dir1/untracked
+;	dir2/modified
+;	dir2/untracked
+;	expect
+;	output
+;	untracked
+EOF
+
+test_expect_success "status (core.commentchar with submodule summary)" '
+	test_when_finished "git config --unset core.commentchar" &&
+	git config core.commentchar ";" &&
+	git status >output &&
+	test_i18ncmp expect output
+'
+
+test_expect_success "status (core.commentchar with two chars with submodule summary)" '
+	test_when_finished "git config --unset core.commentchar" &&
+	git config core.commentchar ";;" &&
+	git status >output &&
+	test_i18ncmp expect output
+'
+
 cat > expect << EOF
 # On branch master
 # Changes not staged for commit: