added annex.commitmessage

Added annex.commitmessage config that can specify a commit message for the
git-annex branch instead of the usual "update".

This commit was supported by the NSF-funded DataLad project.

doc/git-annex.mdwn

@@ -974,6 +974,17 @@ Here are all the supported configuration settings.
   since it could garbage collect objects that are staged in git-annex's
   index but not yet committed.
 
+* `annex.commitmessage`
+
+  When git-annex updates the git-annex branch, it usually makes up
+  its own commit message ("update"), since users rarely look at or
+  care about changes to that branch. If you do care, you can
+  specify this setting by running commands with
+  `-c annex.commitmessage=whatever`
+
+  This works well in combination with annex.alwayscommit=false,
+  to gather up a set of changes and commit them with a message you specify.
+
 * `annex.merge-annex-branches`
 
   By default, git-annex branches that have been pulled from remotes

doc/todo/be_able_to_specify_custom_commit_message_for_git-annex_branch_commit.mdwn

@@ -1,3 +1,5 @@
 ATM many commit messages in git-annex branch are just an "update".  But I wondered if it could be relatively simple to implement to be able to provide a custom commit message, e.g. by defining some environment variable (e.g. GIT_ANNEX_UPDATE_MESSAGE) during whatever process is initiating the actual commit?  sure thing there could be cases whenever multiple processes "contributed" to the changes which are finally "flushed" by committing, but imho it could provide a good way to annotate changes within git-annex branch at least to some degree.  E.g. I could use "update: added URLs to the files for datalad-archives remote" ;)
 
 [[!meta author=yoh]]
+
+> [[done]] as annex.commitmessage config. --[[Joey]]