drop support for Git versions older than 1.7.2.5

This was proposed in #709 and also mentioned on the mailing list.
There have been no objects.

Version 1.7.2.5 is now the oldest supported version.  That is the
version available in Debian oldstable (6.0, Squeeze).

Remove the `magit-configure-have-{*}' functions and `magit-have-{*}'
variables that were used to disable certain functionality for older
Git versions.

In areas affected by this change also sort Git options so that they
are grouped, as much as possible, according to the sections in the
`git-log' man page.
add magit-diff-abbrev-arg and magit-log-cutoff-length-arg

Add `magit-diff-abbrev-arg' and `magit-log-cutoff-length-arg' so we
don't have to repeatably construct the respective Git arguments based on
the values of `magit-sha1-abbrev-length' and `magit-log-cutoff-length'.

Also always use "-1" not "--max-count=1" or ("-n" "1") to turn `git log'
into a command that shows information about exactly one commit.  Git
should really have such a command, which is as powerfull as `git log -1'.
Well there is not command `git log-1' but `git log -1' is close enougth;
consider it a command, `git log --max-count=1' is harder to see as ac
command.
replace some uses of cond with cl-case
add variables magit-{,ref}log-format

Add variables `magit-log-format' and `magit-reflog-format' replacing
the variables `magit-git-log-options' and `magit-git-reflog-options'.

The old variables contained the same "--format:format=..." and
additional the value of function `magit-diff-abbrev-arg'.  These
variables were set at startup.  Changing the value of option
`magit-sha1-abbrev-length' and therefor `magit-diff-abbrev-arg' was
not taken into account.  When the `custom-file' was loaded before
`magit' then even the initial value was wrong.
fix magit-refresh-reflog-buffer inconsistencies

Rename the argument REV (revision) of `magit-reflog' and
`magit-refresh-log-buffer' to REF (reference).  The reflog
deals with branches not with arbitrary revisions.

Remove `magit-refresh-reflog-buffer's ARGS argument.  Its name
was wrong it wasn't a list but an atom.  The proper name would
have been something along the line RESOLVED-REF.  Remove the
argument entirely because we might as well evaluate
`(magit-rev-to-git ref)' inside `magit-refresh-reflog-buffer'
instead of `magit-reflog'.