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>

cache.h

@@ -562,6 +562,12 @@ extern int core_preload_index;
 extern int core_apply_sparse_checkout;
 extern int precomposed_unicode;
 
+/*
+ * The character that begins a commented line in user-editable file
+ * that is subject to stripspace.
+ */
+extern char comment_line_char;
+
 enum branch_track {
 	BRANCH_TRACK_UNSPECIFIED = -1,
 	BRANCH_TRACK_NEVER = 0,