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'.
magit-item-highlight: use definition similar to that of highlight

When `magit-diff-use-overlays' is non-nil set the default value of face
`magit-item-highlight' to something similar to the default value of
`highlight'.  Do not inherit from `highlight' but instead copy the
value (with some redundancy removed), because we want to use a blue
instead of green background in the "very few colors" case.  Green is
already used in diffs for "added lines".  Also this potentially avoids
problems with themes.
magit-log-display-author-date: adjust margin in correct window

This partially reverts f30e76945f82602622a5023c1ffca4dd6ef6d0ee.
Before that commit we were not "unnecessarily retrictive" in making
sure *current buffer* was the correct one.  Instead made sure that
the correct *window* was selected.

The change to `magit-log-create-author-date-overlay' was correct.
magit-log-{initialize,create}-author-date-overlay: move mode check to caller

It's better to do that there so that the reader of the caller knows
right away that these functions don't do anything (now are not called)
unless the mode derives from `magit-log-mode'.
add magit-log-create-author-date-overlays

Merge functions `magit-log-set-author-date-overlays' and
`magit-log-create-author-date-overlay' (singular (wrong))
into new `magit-log-create-author-date-overlays'.

Old `set' was only used by old `create'.
Also what does "set" supposed to mean?
rename magit-log-{display => refresh}-author-date

That's the term we commonly use for this action.
rename magit-log-initialize-author-date-overlay to magit-log-setup-author-date

Rename `magit-log-initialize-author-date-overlay' to
`magit-log-setup-author-date', removing the "-overlay" suffix (this
function doesn't touch any overlay) and replacing "initialize" with
"setup" (which is also not optimal but since we use the same term in
`magit-mode-setup' it makes at least some sense).
magit-log-author-{*} variables: remove useless doc-strings

Better no doc-string than one that provides no useful information.
move misplaced custom options

Previously the options `magit-merge-warn-dirty-worktree' and
`magit-log-author-date-max-length' were defined somewhere near their
use.  That's wrong because I have a section which is supposed all
options.  Move them there.  True many options are misplaced *inside*
that section, but that is a different issue.
magit-log-author-date-max-length: improve doc-string
magit-log-create-author-date-overlays: cosmetics
magit-insert-staged-changes: fix regression

Fix regression in `magit-insert-staged-changes' that caused
staged changes not to be displayed on pristine branches.
quiet byte compiler
magit-commit-at-point: never raise error

Remove the NOERROR argument and just *never* raise an error if
there isn't a commit at point.  That's what most callers need.
magit-guess-branch: remove unnecessary kludge

`magit-section-case' now always returns nil if no section matches,
so we no longer have to deal with a bogus `t' return value.
magit-guess-branch: remove unnecessary complication