Refuse to upgrade direct mode repositories when git is older than 2.22

That git fixed a memory leak that could cause an OOM during the upgrade.

Most git-annex builds have a new enough git already.
OSX git was upgraded with brew.

Linux i386ancient build's git was too old. Upgrading it to a fixed
git didn't work (due to the newer git not working with the old ssh,
https://bugs.chromium.org/p/git/issues/detail?id=7 )

Choices to deal with that were:

* Somehow make direct mode upgrade work with the old git, avoiding its
  OOM problem. One way would be to switch the repo to indirect mode
  first, and so upgrade to a repo with locked files. Not good when
  the filesystem does not support symlinks.
* backport the OOM fix from git 2.22
  (And do what about the version number so git-annex knows it's fixed?)
* backport openssh (and possibly more stuff)
* move the i386ancient build to at least Debian stretch (still backporting git)
  But this will make it no longer work with some of the ancient kernels it
  targets.

Of those, backporting the OOM fix seemed the best approach. Put "oomfix"
in the git version number to indicate it.

I have not automated building the git backport, so here's the patch I
used:

diff -ur orig/git-2.1.4/convert.c git-2.1.4/convert.c
--- orig/git-2.1.4/convert.c	2014-12-18 18:42:18.000000000 +0000
+++ git-2.1.4/convert.c	2019-08-29 20:05:04.371872338 +0100
@@ -404,7 +404,7 @@
 	if (start_async(&async))
 		return 0;	/* error was already reported */

-	if (strbuf_read(&nbuf, async.out, len) < 0) {
+	if (strbuf_read(&nbuf, async.out, 0) < 0) {
 		error("read from external filter %s failed", cmd);
 		ret = 0;
 	}
diff -ur orig/git-2.1.4/GIT-VERSION-GEN git-2.1.4/GIT-VERSION-GEN
--- orig/git-2.1.4/GIT-VERSION-GEN	2014-12-18 18:42:18.000000000 +0000
+++ git-2.1.4/GIT-VERSION-GEN	2019-08-29 20:06:39.132743228 +0100
@@ -1,7 +1,7 @@
 #!/bin/sh

 GVF=GIT-VERSION-FILE
-DEF_VER=v2.1.4
+DEF_VER=v2.1.4.oomfix

 LF='
 '
diff -ur orig/git-2.1.4/configure git-2.1.4/configure
--- orig/git-2.1.4/configure	2014-12-18 18:42:19.000000000 +0000
+++ git-2.1.4/configure	2019-08-29 20:27:45.896380015 +0100
@@ -580,8 +580,8 @@
 # Identity of this package.
 PACKAGE_NAME='git'
 PACKAGE_TARNAME='git'
-PACKAGE_VERSION='2.1.4'
-PACKAGE_STRING='git 2.1.4'
+PACKAGE_VERSION='2.1.4.oomfix'
+PACKAGE_STRING='git 2.1.4.oomfix'
 PACKAGE_BUGREPORT='git@vger.kernel.org'
 PACKAGE_URL=''

diff -ur orig/git-2.1.4/version git-2.1.4/version
--- orig/git-2.1.4/version	2014-12-18 18:42:19.000000000 +0000
+++ git-2.1.4/version	2019-08-29 20:06:17.572545210 +0100
@@ -1 +1 @@
-2.1.4
+2.1.4.oomfix

doc/bugs/git-annex_upgrade_fails.mdwn

@@ -43,3 +43,5 @@ Would be happy if anybody could share some insight on how to get that repo to v7
 
 Regards,
 Felix
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/git-annex_upgrade_fails/comment_3_acdfcd2bb40820cf69748fe4df956366._comment

@@ -0,0 +1,15 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 3"""
+ date="2019-08-29T17:02:02Z"
+ content="""
+It doesn't seem practical to get around the old git's OOM problem.
+I think that the best thing to do is to not allow direct mode upgrade to v7 
+until git has been upgraded.
+
+The minimum git version is 2.22. Unfortunately, Debian stable shipped 2.20.
+So direct mode users there who upgrade git-annex will need to also upgrade
+git in order for git-annex to convert their repo -- and since the next
+git-annex release drops support for direct mode, they'll have to upgrade
+git (or downgrade git-annex) in order to use git-annex in their repo at all.
+"""]]