test: prepare tests for removing date aliases

In the next patch, I'm gonna removing the global command aliases that force
the epoch date but some tests either fail or their output change after that.
Instead I'm copying the needed aliases in the test files that will otherwise
change.

Update test-rebase-obsolete.t because a revision hash is based on the epoch
date after a 'commit --amend' and the output will change after removing date
aliases.

Update test-subrepo-git.t as the git subrepo doesn't use traditional date
mechanisms. I'm not sure that updating the git subrepo to support default-date
make sense. Add the commit alias to the test in order for making it pass after
removing the date aliases globally.

tests/test-rebase-obsolete.t

@@ -902,7 +902,7 b' rebase source is obsoleted (issue5198)'
   $ hg up 9520eea781bc
   1 files updated, 0 files merged, 2 files removed, 0 files unresolved
   $ echo 1 >> E
-  $ hg commit --amend -m "E'"
+  $ hg commit --amend -m "E'" -d "0 0"
   $ hg log -G
   @  9:69abe8906104 E'
   |

tests/test-subrepo-git.t

@@ -2,6 +2,11 b''
 
 make git commits repeatable
 
+  $ cat >> $HGRCPATH <<EOF
+  > [defaults]
+  > commit = -d "0 0"
+  > EOF
+
   $ echo "[core]" >> $HOME/.gitconfig
   $ echo "autocrlf = false" >> $HOME/.gitconfig
   $ GIT_AUTHOR_NAME='test'; export GIT_AUTHOR_NAME